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
Do not activate FakeBraintree when loaded

We want to activate manually instead. This is necessary when running
tests in multiple processes.