Update generator to yeoman-generator 0.17

TODOs:

- [ ] Tests failures due to https://github.com/yeoman/generator/issues/614
- [x] Need to update (more) to using File-utils and not the legacy methods
- [x] Need to update to the new tests utilities (both template and test code - test code is pretty raw, it'd be great to test more thoroughly)
- [x] Run tests in a tmpdir rather than in `./tmp`
- [x] generate `.yo-rc.json` file (require tmpdir fix)
- [x] Update to using `assert` assertion rather than the deprecated `helper.assertX`
I guess this can be done in a followup :)