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
Is the NullPointerException being thrown  by `url.getPath()`? If so I think it would be better to add a precondition check before this block, rather than try-catch. Better to avoid using a null than to catch a null pointer exception.