Add python 3.4 support

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:
* 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