Python 3.x compatibility fixes

This fixes a couple of things to make sure the lib runs on Python 3 (tested on 3.4.2):

* Implicit relative imports (`import clockwork_http`) have been made explicit (`from . import clockwork_http`)
* Mixed use of indentation in one file (spaces and tabs), everything is converted to 4 spaces.
* `Urllib2` does no longer exist in Python 3 and is split in `urllib.request` and `urllib.error`. Fixed imports accordingly in a backward compatible way.