add support for basic Releases methods

include Repositories.ListReleases, Repositories.GetRelease,
Repositories.CreateRelease and Repostories.DeleteRelease.
add support for Release assets

Includes Repositories.ListReleaseAssets, Repositories.GetReleaseAsset,
Repositories.EditReleaseAsset, Repositories.DeleteReleaseAsset,
Repositories.UploadReleaseAsset.
add Thomas Bruyelle as contributor
Merge remote-tracking branch 'upstream/master' into list-watchers

* upstream/master:
  add Thomas Bruyelle as contributor
  add support for Release assets
  add support for basic Releases methods

Merging with upstream