Add python 3.4 support

Howdy, This is more to start a conversation than a complete PR. Ultimately I wanted to know if there was a non zero chance of getting merged before I went further.

I find this tool really cool and thought it could benefit by supporting python3. I went ahead and tried to give it python3 support. This PR gets pretty close to that.

I figured it may be a long shot as edX is a python2 shop but whatever, I thought I would give it a shot.

Here is the TODO:
*Currently this port requires this patch to apply pygments_patch.patch I hate this... (see https://bitbucket.org/birkenfeld/pygments-main/commits/c31841d2fd21#chg-pygments/lexers/__init__.py) this will fix itself when this library gets updated.

*The python26 and 27 seem to have a couple tests that fail sometimes due to ordering. Any ideas there btw? I think it has to do with the file paths being a set.

*Two tests give this error. This might be the deal breaker as I *think* it's caused by the version of pylint required by the project. However, I can try and look into it:
AttributeError: 'TreeRebuilder3k' object has no attribute 'visit_nameconstant'
diff_cover.tests.test_integration.DiffQualityIntegrationTest#test_added_file_pylint_console
diff_cover.tests.test_integration.DiffQualityIntegrationTest#test_added_file_pylint_html