Restore old Credentials API as Obsolete

To make #761 less of a breaking change.

TODO:

* [x] Actually use the `*Options.Credentials` properties.

Currently branched from the HEAD of that PR to make it easier to compare with the previous public API:
https://github.com/libgit2/libgit2sharp/compare/7765c12cf3315a0d8c3d28f61a261510abf348e2~...dahlbyk;ObsoleteCredentials
> Travis seems unhappy with the `LibGit2SharpInterfacesCoverAllPublicMembers()` meta test though.

Not sure how I didn't notice this before... `ICredentialsProvider` is our first non-`public` interface; no need to test it.