Implement optional usage of Futures for parallelizable operations

It would be neat if certain Chain transforms that group multiple other transforms together were able to optionally execute the wrapped transforms in an async fashion.

For example, Tupling and Detupling, as well as maybe Must, Should and MustNot.

See:
- https://pypi.python.org/pypi/futures
- https://pypi.python.org/pypi/trollius
- https://github.com/xogeny/aplus
