Cleanup for https://github.com/peterbe/premailer/pull/86/files#r18591185
Cleanup for https://github.com/peterbe/premailer/pull/86/files#r18591254
Cleanup for https://github.com/peterbe/premailer/pull/86/files#r18591445
Cleanup for https://github.com/peterbe/premailer/pull/86/files#r18591579
Using the mock.path decorator rather than the context https://github.com/peterbe/premailer/pull/86/files#r18591370