[WIP] Fix unicode support.

This PR adds full support for Unicode in Onitu. Pretty much everything (filenames, options, session name, entries names…) can be Unicode now.

Note that only TestDriver and LocalStorage have been updated to support Unicode. We still need to update the other drivers.

- [x] Core
- [x] Tests
- [x] TestDriver
- [x] LocalStorage
- [x] S3
- [x] Dropbox
- [ ] Hubic