Enable the usage of existing 'salt-call -m' option.

Currently, the ``-m`` flag for salt-call has no effect in the loaders.
This commit allows users to specify exactly the module dirs to load
given that they follow the naming conventions for 'grains', 'runners',
'modules', 'states', etc. This is a shortcut for having to refresh cache
state when using a masterless salt-call.
Updated old README with community content

As xt pointed out, this was a bit dated. I pulled replacement content
from the first paragraph in the docs and the community page on the
website.
Fix several docstrings
Catch possible traceback when passing incorrect lowstate to wheel system
XCP <= 1.6 should use yumpkg5
Merge pull request #4261 from whiteinge/salt-readme

Updated old README with community content
Use get to avoid an error when the function loses the retcode

Fix #4267
protect from making / absent
Merge pull request #4270 from dlindquist/develop

Fix several docstrings
Add verification for names to be a list, fix #4265
Fix a few RST problems
Adding tomcat state documentation

Fixes #4256
Add a log message to when a bad renderer is passed in

Fix #4269
Merge pull request #4271 from whiteinge/salt-wheel-traceback

Catch possible traceback when passing incorrect lowstate to wheel system
That was embarrasing, I left the self in...

Fix #4275
Clean up log print statement that was left behind
Add XCP to the os_family grainmap per #4277
Merge pull request #3976 from dcolish/salt-call-use-cli-arg-modules

Enable the usage of existing 'salt-call -m' option.
Use os_grain instead os_family.  Also, use yumpkg for XCP >= 2.0 per http://wiki.xen.org/wiki/XCP_Roadmap
Merge pull request #4277 from dlindquist/xcp

XCP <= 1.6 should use yumpkg5