Add demo for Monte Carlo estimation of pi

See examples/pi-montecarlo/README.rst for explaniton.
Add client & view as attributes to context. Specify execptions.
Tweaks to pi_montecarlo example.

* Tweaks imports to make parallels between distarray and numpy clear.
* small formatting touchups.
* Adds simple @timer decorator.
Add docstring to benchmark script.
Remove benchmark plot.
Change to reflect recent API changes.
pep8