Add modules to install gitfs backend dependencies

Add modules for gitfs dependencies and documentation for how to use them.

README.rst

 Install a minion and configure it in `standalone mode
 <http://docs.saltstack.com/en/latest/topics/tutorials/standalone_minion.html>`_.
 
+``salt.gitfs.dulwich``
+----------------------
+
+Install gitfs backend dulwich dependencies. Set ``salt:master:gitfs_provider: dulwich`` in your pillar.
+
+``salt.gitfs.gitpython``
+----------------------
+
+Install gitfs backend GitPython dependenciess. Set ``salt:master:gitfs_provider: gitpython`` in your pillar.
+
+``salt.gitfs.pygit2``
+----------------------
+
+Install gitfs backend libgit2/pygit2 dependenciess. Set ``salt:master:gitfs_provider: pygit2`` in your pillar.
+
 ``salt.pkgrepo``
 ----------------
 

salt/defaults.yaml

   salt_api: salt-api
   salt_ssh: salt-ssh
   clean_config_d_dir: False
+
+  master:
+    gitfs_provider: gitpython
+
+  gitfs:
+    dulwich:
+      install_from_source: True
+    pygit2:
+      install_from_source: True
+    gitpython:
+      install_from_source: False

salt/gitfs/dulwich.sls

+{% from "salt/map.jinja" import salt_settings with context %}
+# issue 34
+
+{% if salt_settings.gitfs.dulwich.install_from_source %}
+# we probably don't have a package or it's not a high enough version
+# install latest from source/pip
+dulwich-deps:
+  pkg.installed:
+    - pkgs:
+      - build-essential
+      - python-dev
+
+install-dulwich:
+  pip.installed:
+    - name: dulwich
+
+{% else %}
+# install from package
+# TODO haven't actually found a distro that has a good version to test
+
+{% endif %}

salt/gitfs/gitpython.sls

+{% from "salt/map.jinja" import salt_settings with context %}
+
+{% if salt_settings.gitfs.gitpython.install_from_source %}
+
+GitPython:
+  pip.installed
+
+{% else %}
+
+python-git:
+  pkg.installed
+
+{% endif %}

salt/gitfs/pygit2.sls

+{% from "salt/map.jinja" import salt_settings with context %}
+
+{% if salt_settings.gitfs.pygit2.install_from_source %}
+# we probably don't have a package or it's not a high enough version
+# install latest from source/pip
+pygit-deps:
+  pkg.installed:
+    - pkgs:
+      - build-essential
+      - python-dev
+      - libssh-dev
+      - libffi-dev
+      - cmake
+
+dl-libgit2-src:
+  archive.extracted:
+    - name: /usr/src
+    - source: https://github.com/libgit2/libgit2/archive/v0.22.1.tar.gz
+    - source_hash: md5=dbf516d18e176bbb131de3efccfee533
+    - archive_format: tar
+    - keep: True
+    - if_missing: /usr/src/libgit2-0.22.1
+
+/usr/src/libgit2-0.22.1/_build:
+  file.directory
+
+configure-libgit2:
+  cmd.run:
+    - name: cmake ..
+    - cwd: /usr/src/libgit2-0.22.1/_build
+    - creates: /usr/src/libgit2-0.22.1/_build/Makefile
+
+build-libgit2:
+  cmd.run:
+    - name: make -j4
+    - cwd: /usr/src/libgit2-0.22.1/_build
+    - creates: /usr/src/libgit2-0.22.1/_build/libgit2.so
+
+install-libgit2:
+  cmd.run:
+    - name: make install
+    - cwd: /usr/src/libgit2-0.22.1/_build
+    - creates: /usr/local/lib/libgit2.so
+
+run-ldconfig-after-lib-install:
+  cmd.run:
+    - name: ldconfig
+    - onchanges:
+      - cmd: install-libgit2
+
+install-pygit2:
+  pip.installed:
+    - name: pygit2
+
+{% else %}
+# install from package
+# TODO haven't actually found a distro that has a good version to test
+# debian jessie will have libgit2-21
+
+{% endif %}