Add a few reference utility functions
Allow for a default in reflog messages

We don't have a way to represent a NULL string, so if the user passes an
empty string, let's pass NULL down so we tell libgit2 to use the default.
Merge pull request #60 from libgit2/cmn/reflogs-default

Allow for a default in reflog messages
Merge pull request #59 from libgit2/cmn/refs

Add a few reference utility functions