reindent
add documentation
rewrite getTree and getCommit more simply as we now have simpler object to <type> functions.
throw a proper exception RefInvalid instead of error.
reindent & documentation
add a not found exception for revision.
derive Data/Typeable for ObjectType.
add a getObject_ that raises a proper exception when reference isn't found.
prepare an InvalidType exception for object to Type conversion.
update for hashable 1.2
add a version constraints