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
Are you sure? I thought a `File` object did not have to correspond to a file that actually exists. It has methods on it like `.exists()` and `.mkdirs()`, which would be useless if the file had to exist in order to create the object.