Don't use mutables as default arguments.

Using mutables as default arguments in Python can lead to terrible things. Specifically, if a list is used as a default, and its value is at all changed, the change carries forward to future invocations of the function. More can be read here: http://docs.python-guide.org/en/latest/writing/gotchas/
"Merge pull request #327 from mankyd/patch-1\n\nDon't use mutables as default arguments."
Create utf-8 encoded files in tests
"Merge pull request #331 from rassie/unicode_tests\n\nCreate UTF-8 encoded files in tests"