For SG-9540, upgrade httplib2 (#187)

This upgrades httplib2 to the most recent version (0.12.0). It has been slightly modified so the library imports parts of itself via from . import <something> instead of from http2lib import something, as the library isn't part of the PYTHONPATH.

Tests are also much faster.

Author: jfboismenu

README.md

@@ -36,6 +36,12 @@ Some useful direct links:
 
 You can see the [full history of the Python API on the documentation site](http://developer.shotgunsoftware.com/python-api/changelog.html).
 
+## Updating HTTPLib2
+
+1. Download the latest version of HTTPLib2 at https://pypi.org/project/httplib2.
+2. Extract the python2/httplib2 into shotgun_api3/lib/http2lib without the test folder.
+3. Scan the files for any references to importing httplib2 and make sure they import "from ." instead of "from httplib2" because the library isn't in the Python path.
+
 ## Tests 
 
 Integration and unit tests are provided.