Fix SyntaxErrors on Python 3.2 due to unicode literals

UnicodeLiterals with a leading 'u' are invalid syntax on Python 3, triggering test failures such as those [seen here](https://travis-ci.org/jaraco/pmxbot/jobs/8740138). This pull request brings in the 'u()' and 'b()' functions from six and implements them for unicode literals throughout the codebase (excepting the testsuite package).

I have not tested these commits yet. Please review and let me know if there's more you would like me to do to improve Python 3.2 support.