Test against Ruby 2.0.0
Support custom subscription IDs.

https://www.braintreepayments.com/docs/ruby/subscriptions/create\#subscription_id
All tests passing
Use activesupport < 4.0 when testing Ruby <1.9.3

Travis CI builds were failing on 1.9.2 because the Gemfile tried to install
activesupport 4.0.0+, which requires Ruby 1.9.3+.
adding support for BT submit_for_settlement call