Updates README to reflect all updates to module

Removes references to capabilities the module no longer has. Edits
sentences for capitalization, spelling, and grammar errors. Edits
sentences for contextual consistency.
Updates links in README.
Merge pull request #120 from laurenrother/readme

Updates README to reflect what the module is currently doing
Allow custom gemsource
Merge pull request #123 from hunner/dynamic_gemsource

Allow custom gemsource