v0.13.0

This release:

• Adds the git ops for listing pull requests (#70)
• Includes the ops for creating tags and references (#71)
• Changes the RepoStatus model, making optional these fields:

--    open_issues: Int,
--    watchers: Int,
--    network_count: Int,
--    subscribers_count: Int,
++    open_issues: Option[Int],
++    watchers: Option[Int],
++    network_count: Option[Int],
++    subscribers_count: Option[Int],