Add options for remove_groups and default to false

https://github.com/saltstack/salt/issues/2142

This has been causing issues for many people, as the remove_groups options is undocumented.  In the 2014.7 release this is changing, and remove_groups will default to false:
https://github.com/saltstack/salt/issues/13276

I'm going with false by default, as it's our use case and it will soon be the default.  If people believe this module should default to true and remove groups not listed, I think that's open for discussion, but we should at least add the option.

Change default shell to /bin/csh on FreeBSD

This also made it necessary to introduce an additional entry 'shell' into the
users lookup table as the formula previously conflated the shell used for
running the visudo command and the default shell to be used for user accounts.

Fixes: #48

Re-add ability to set shell per-user

Add 'createhome' option for 'user.present' state

Fix issue with bash binary location in FreeBSD

special case for root user

modified visudo to only report change in salt when there is an error. also add option to remove keys from ssh_auth

Only include users.sudo when a user with sudouser is declared.

Support Gentoo minions

Add support FreeBSD using map.jinja (Tested on Freebsd10)

Specify bash shell when validating sudo snippet

Since we are using bash specific features and sometimes you can end up getting /bin/sh - see https://github.com/saltstack-formulas/users-formula/pull/30#issuecomment-44224106

Overwrite a sudoer file rather than append to fix #21

Do not echo Public/Private keys to stdout when running the state

Ensure that we are sudo includes /etc/sudoers.d

It doesn't by default everywhere - without this the sudo_rules option won't work

Validate user sudo rules before applying them

Add Password if the state supports it.

corrected url for formulaes...

New format of handling user.absent, supporting additional parameters

Fix incorrect sudo_rules example

Add home to pillar.example

Fix pubkey privkey typo

Fixes #22

Add absent_groups pillar to remove groups.

Updated structure of readme, bumped License to 2014.