Python 3 compatibility. (#121)

* Make common module Python 3 compatible.

* Make plasma module Python 3 compatible.

* Make photon module Python 3 compatible.

* Make numbuf module Python 3 compatible.

* Remaining changes for Python 3 compatibility.

* Test Python 3 in Travis.

* Fixes.

lib/python/photon/__init__.py

@@ -1 +1 @@
-from photon import *
+from .photon import *

lib/python/photon/photon/__init__.py

@@ -1,2 +1,6 @@
-from libphoton import *
-from photon_services import *
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+from .libphoton import *
+from .photon_services import *