Account Store CRUD

I've added the Account Store CRUD and written tests to cover the specification. 

The one thing I would like to consult with you is the fact that in the specification there was a part "in order to return a proper instance of an account store some change to the core implementation will need to be made to read from the account store href".

I did not change the associations core implementation, since i think this is a really specific case in the whole API, and the approach that i did ( https://github.com/DamirSvrtan/stormpath-sdk-ruby/compare#diff-0cfa27575ee4d1468770066333475d18R8) seems to me much cleaner than making some polymorphic interface such as AccountStore. Please tell me your opinion.