Add a little HTTP request resilience

At times (more often than I would like) box.com API services [experience connectivity issues](http://status.box.com). (And at times a pesky OS X 10.9 [network stack bug](https://discussions.apple.com/thread/5551686) gets in the way.) The occasional symptoms are hung sockets and SSL EOL/EOF errors.

I worked-in a requests session HTTP adapter that puts timeout values of requests and retries requests that fail.

With some massaging, could this mechanism be part of box.py?