Add support for blocking pools

[scott-8]

Right now, a RunetimeError is raised is no connections are available in a pool. It would be helpful if there were an option to block the thread instead. Redis and pylibmc have examples of this.

[JoJo10Smith]

@jogo I'd like to try and work on this issue but I have a few questions about the function:

• Should I be creating a new function or just adding the option to block in the get() function in pool.py?

I could submit a PR sometime this week with my potential solution based on the feedback to my questions above.
Thanks

[jogo]

sorry for the slow response, I think a new option in get would be preferable.