add support for basic Releases methods

include Repositories.ListReleases, Repositories.GetRelease,
Repositories.CreateRelease and Repostories.DeleteRelease.
Support for Repository Contents API
add support for Release assets

Includes Repositories.ListReleaseAssets, Repositories.GetReleaseAsset,
Repositories.EditReleaseAsset, Repositories.DeleteReleaseAsset,
Repositories.UploadReleaseAsset.
Merge remote-tracking branch 'tbruyelle/Releases'
Merge remote-tracking branch 'octavore/repo-contents'