Issue #214: Cannot call TreeEntry() / IndexEntry()
Fix a couple of compilation warnings
travis: we are back to use libgit2's master branch
Add GIT_OID_ constants

- GIT_OID_RAWSZ
- GIT_OID_HEXSZ
- GIT_OID_HEX_ZERO
- GIT_OID_MINPREFIXLEN
create_reference, drop the symbolic param (#213)

Now we figure out whether it is a direct or symbolic reference based on
the type and value of the target. This guessing will fail in very very
rare situations, for that we still have the explicit lower level API.