Finish Balanced Payments Support.

Fixes #63
renamed Balanced to BalancedGateway to avoid namespacing issue with Balance gem for their API

Fixes #62
added test for BalancedGateway

Related to #62
Rename Stripe gateway to StripeGateway

Fixes #60
Specify spree as a dependency in Gemfile (so that we can use 2.0.0)