Introduce default credentials

Abstract Credentials, providing UsernamePasswordCredentials
and DefaultCredentials.
Introduce EmptyCommitException

By default, we now throw an exception when trying to commit an
"empty commit" - that is, one that does not change any items.
This check only applies when doing a normal commit, or an amend
of a normal commit; it does not apply when performing a merge or
an amend of a merge commit.  This check can optionally be overridden.
Introduce CommitOptions

Repository#Commit (and its extension methods) now take CommitOptions.
All prior methods are deprecated.