Added documentation page about using git.

reference: bug 867558
This documentation page explains how to setup GitHub and git and also how to use them during development.  Used some examples from Socorro development.  rhelmer thought this page would be a good idea for newcomers.  Still need to update development.rst to add this page to the index.
fixes bug 848680 - add pig support to crontabber
Adding a few tests for new ESR numbering scheme
Fixed minor Sphinx formatting issues + minor edits

reference: bug867558

Fixed some Sphinx formatting issues involving backticks.  Also edited a few phrases and reordered a sentence for clarity.
Fixes bug 864752 - Excluded channels now begin with the value in configuration.
Merge pull request #1238 from selenamarie/test-for-new-esr-versions

Adding a few tests for new ESR numbering scheme
Merge pull request #1243 from AdrianGaudebert/864752-excluded-channels-start-with

Fixes bug 864752 - Excluded channels now begin with the value in configu...
Merge pull request #1213 from terriyu/bug867558-doc-git

Added documentation page about using git.
Merge pull request #1121 from peterbe/bug848680-add-pig-support-to-crontabber

bug 848680 - add pig support to crontabber