Refactored Executable, Promise, Future, etc. Due to the Swift exception paradigm, Promise now contains a non-Try value. Promise and Future should be parametrized on Try<T> should the user wish to capture the notion of a failure. Removed appropriate Promise methods involving Try or exceptions. Awaitable now captures a timeout in a closure to generalize the notion of timing out.
Added List<T> constructors (^, ^^, ^&^)
Added append functionality
Removed unused OnceExecutable.
Try<T> now captures .Failures of the protocol TryFailure, of which NSException adopts.