added branchOn combinator

[ludicast]

The branchOn combinator lets you put some logic in the class declaration.

requestSoda:
  locateWaiter \
  placeOrder \
  branchOn(customerBMI)
    success: bringJumbo
    failure: callCopsOnLardass

I'll add documentation for any of these orphaned combinators your repo adopts (I always TDD them so the tests demonstrate usage).