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
It looks like the third argument here is the project type - would it be appropriate to change the type from String to ProjectType so that we can't accidentally send e.g. "Fiel"? I guess we would need to send invalid values to test the error behaviour, so it is probably better to leave it as-is.