Add file tests, clean up and simplify

- Test html, idml, dtd, odg/p/s/t File type upload
- Use ZanataRestCaller rather than manually creating projects, versions
- Remove unused imports
- Add a null pointer check on opening files
The API key here is a bit of a magic value - if it appears in multiple tests then we could pull it out into a constant somewhere, which would make it easier to update if we change the translator API key some time.