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
fix rspec deprecation warnings
add urls to token
Add merchant_acount api
work on client api