How to fix the "No Rack handler defined" error
Add specs to test customer and subscription retrieval by custom integer id
implement id normalization for custom ids, which can be provided upon customer and subscription creation.
fixes issue where retrieval of entity is not possible if it was stored with an integer as id.
Update NEWS.md
Always use WEBrick as the Capybara server

Thin has a number of problems as the Capybara server handler:

* https://github.com/thoughtbot/fake_braintree/issues/54
* https://github.com/jnicklas/capybara/pull/920

Capybara >= 2.0.2 can directly use WEBrick as its server handler, and in fact is
preferred (see link below). It works and shouldn't have the same threading
issues. WEBrick is also more convenient, since it's included in Ruby.

Link: https://github.com/jnicklas/capybara/blob/master/History.md#version-202
Require Ruby 1.9.3+

We were allowing Ruby 1.9.2, which is EOL'd.
Move all `require`s into fake_braintree.rb

Remove unnecessary requires, like requiring `capybara` and `capybara/server`.
Remove advice that no longer applies

fake_braintree no longer uses Thin or Puma in favor of Webrick.
Document FakeBraintree.registry.transactions
Remove stray debugging statement
Save discounts amount when adding to Subscription
Update NEWS.md for #72
v0.6.0
WIP: Braintree::ClientToken.generate