[WIP] Add country choice form widget

This is a first stab at adding a different form widget, split into number and country choice (refs #19).

![image](https://cloud.githubusercontent.com/assets/1784740/4875973/cfc6f530-62af-11e4-8687-5080a1559bea.png)

The country choice is customisable (loads all from libphonenumber by default), and is translated. Loading a large list is rather memory intensive (the whole list is about 3MB), so it might be better to cache this data when the container is compiled.

- [ ] Add tests
- [ ] Look into caching valid region codes