Add Repository.Stashes.Add
Add Repository.Stashes.GetEnumerator()
Add implicit cast from GitOid(?) to ObjectId

Also, support marshaling NULL in OidSafeHandle
Use implicit GitOid to ObjectId cast everywhere
Give props to Jon Skeet

Add link back to source Stack Overflow post for the CopyStream
method after obtaining permission from Jon Skeet to include in
LibGit2Sharp.
Throws when HEAD is missing

Fix #351
Revert "Remove useless GC.SuppressFinalize()"

This reverts commit d03bcb995993867f9260f32786ca25d6071d563d.

cf. http://msdn.microsoft.com/en-us/library/ms182269.aspx