Firefox Account module update

Comoyo's branch need to be rebased. To see the changes:

https://github.com/borjasalguero/gaia/commit/80c997f5dd9271d823ecdfb394b897313f9970d9

This PR contains:
- Adapt JS structure to the namespace agreed in Madrid
- Added communication with 'System'
- JS architecture for the module
- Common markup duplications removed.


TODO's list:
- Need to restore animations
- Need to review 'View' & 'Step' Objects for all scenarios