Update default media type to v3

See http://developer.github.com/changes/2014-01-07-upcoming-change-to-default-media-type/
Remove deprecations

Removes support for some deprecated method arguments.
Version bump
Merge pull request #399 from octokit/v3-media-type

Update default media type to v3
Remove deprecated method #create_download.
Merge pull request #401 from joeyw/remove-deprecated-download-method

Remove deprecated method #create_download.