OdbBackend: initial pass on Python subclassing

The basic approach here is to have backends created from C (loose, pack)
circumvent the OdbBackend_init function. However, backends created from
Python will not. We then use this backend to pull out the functions
implemented in Python and rig them up through shim functions to the
libgit2 git_odb_backend.

Author: ddevault

src/odb.c

@@ -158,7 +158,7 @@ Odb_read_raw(git_odb *odb, const git_oid *oid, size_t len)
     int err;
 
     err = git_odb_read_prefix(&obj, odb, oid, (unsigned int)len);
-    if (err < 0) {
+    if (err < 0 && err != GIT_EUSER) {
         Error_set_oid(err, oid, len);
         return NULL;
     }
@@ -288,7 +288,7 @@ Odb_add_backend(Odb *self, PyObject *args)
     }
 
     err = git_odb_add_backend(self->odb, backend->odb_backend, priority);
-    if (err > 0)
+    if (err != 0)
         return Error_set(err);
 
     Py_RETURN_NONE;