Python-pmap

New
- `pvalmap` and `pkeymap` functions to map over the values and keys of a dictionary.

Bugfix
- `pmap` need to start work on thread pool unconditional of `deref` call.

New
- `Future` now takes a `pool` argument to specify a thread pool.
- `Future` now implements `__call__` to allow `Future(f)("foo", k=42)` style to pass additional arguments.
- `Future.deref()` now accepts an optional `timeout` parameter to avoid blocking forever.
- Introduce `Deferred` object to encapsulate deferred results and re-throwing with original traceback.

New
- Keep original traceback when re-throwing errors on realization of `pmap` generator.

New
- Introduce `pmap` function for mapping over collection (lazily) in parallel.