Updated links in contributing guidelines
Updated documentation for Issues methods
Use PATCH instead of POST when updating things

Brings all updates inline with the GitHub documentation
Added check_assignee method to Repositories

As outlined here:
http://developer.github.com/v3/issues/assignees/#check-assignee
Merge pull request #216 from andrew/issue-docs

Updated documentation for Issues
Merge pull request #217 from andrew/patch-over-post

Use PATCH instead of POST when updating things
Merge pull request #218 from andrew/check-assignee

Added check_assignee method to Repositories