Oid: Deprecate 'hex' in favour of str() or unicode()

Python has built-in functions to get a string representation of an
object id, so let's use that instead of using a custom attribute.
Object: move to use an 'id' attribute instead of 'oid'

This looks like a left-over from the libgit2 misnaming. The current
consensus is that 'oid' is the data type and 'id' is the name of the
attribute.
TreeEntry: move to use 'id' attribute for the object's id

Similar to the Object change, we should be using 'id' when referring to
the target's id.x