Salt writes it's schedule file to /etc/salt/{minion,master}.d/_schedule.conf

We don't want to stomp all over Salt's files, but we do want a pristine
starting point to lay down our managed config. So we use clean: True on the
file.recurse call, but we tell it to ignore files that start with an _

We have to rename the current config file (_defaults.conf) because it will be
ignored by the rule that ignores Salt's _* config files.

This also means we need to clean up old config files (_defaults.conf) and
restart the service if we cleaned it up.

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

If you are installing Salt via git/pip, the formula will try to overwrite your
install with packaged versions. This setting makes it possible to avoid that.

New versions of Salt put config files in /etc/salt/{minion,master}.d. We don't
want to erase them by using a clean: True on the file.recurse. This is a
backward incompatible change, but it's necessary to avoid deleting Salt config
files.

Resolves #104

Using a variable named salt is a bad idea when salt already has a variable
named salt.