Work around circular ref in Python stdlib's 'urlopen'

The Python standard library's `urlopen` function (`urllib2.urlopen` in Python
2, and `urllib.request.urlopen` in Python 3) has a circular reference problem.
This patch provides a simple helper method, 'http_get', which fetches text from
a URI and which works around this issue. Verified with gc.DEBUG_LEAK.

The leak and some workarounds are documented in <http://bugs.python.org/issue1208304>.

Change-Id: I9961c7a3769e0827ad5d83eaa10b1911182b2904