Remove log statements from Operation.
Operations can now return a new operation to execute in its place.

This commit adds a new StepResult AddThen(op) which an operation
can return to indicate that a new operation should be executed and
its result returned as the return of this operation.
Add test for AddThen() StepResult.
Remove val from Op2 arguments.
Add "andThen" as a synonym for Operation.flatMap().
Merge pull request #458 from joshmarcus/topic/andthen

An operation can now return an operation to run in its place.