Rails-friendly initialisation, refactoring, error handling

Qutonic's fork edition:
* Move configuration to be a hash passed to the top-level Pin module
* Refactor to move most API object logic into base
* Add an exception and throw it on API error
* Support Pin.js and a publishable key for payment forms
* Document the above
Merge pull request #2 from qutonic/master

proposing a refactor, better error handling, and initialisation changes