Added initial support for BBVA bank (only get_balance supported)

This PR is the first port of the [BBVA](https://github.com/javiercr/bbva) gem.

The major changes are:
* Uses curb instead of Faraday
* Uses new BBVA API (from the latest version of the mobile app), which uses JSON responses instead of XML.

Work for the future:
* Implement transactions method
* Support for multiple bank accounts