Add pure Python fallback module

Signed-off-by: Bas Westerbaan <bas@westerbaan.name>

Author: bwesterb

msgpack/__init__.py

@@ -1,8 +1,12 @@
 # coding: utf-8
 from msgpack._version import version
 from msgpack.exceptions import *
-from msgpack._packer import pack, packb, Packer
-from msgpack._unpacker import unpack, unpackb, Unpacker
+
+try:
+    from msgpack._packer import pack, packb, Packer
+    from msgpack._unpacker import unpack, unpackb, Unpacker
+except ImportError:
+    from msgpack.fallback import pack, packb, Packer, unpack, unpackb, Unpacker
 
 # alias for compatibility to simplejson/marshal/pickle.
 load = unpack