Add support for using requests

- Enable the use of keep-alive (managed by sessions) to streamline a
    large number of API requests into a single HTTP connection for much
    improved performance.

I also have this implemented as an optional import.  That said, it might be wise to just drop support for urllib2, as updating 200+ starbases is much faster if all the API requests reuse the same HTTP connection (about 5-7x, it's _very_ significant).