Adds an optional source
Rename, remove references to Sink
Merge pull request #963 from twitter/optionalMappable

Adds an optional source
Make TypedPipe immutable
Add FlowStateMap support to get REPL tutorial running again
Get Mode out of where it is not needed, and put it where it is
Minor formatting fixes
Memoize inside ContinuationTypedPipe
Fix bug in repl test, change ContinuationTypedPipe to TypedPipeFactory
Add NoStackAndThen to make sure we never blow the stack
Fix 2.10 compile failure
Remove a couple of places where we could get stack overflows
Address review comments
Fix typo in comment
Add the Execution Monad
Get the tmp directory from the config
Merge pull request #968 from twitter/ref_trans_typedpipe

Make TypedPipe immutable
Add some tests
Generalize handling of merged TypedPipes
Add more zip methods and .unit to ease composition