Feature/remote web driver

Experimental support for running Storyplayer against a copy of Selenium that's running who-knows-where.

This addresses #51, and makes it possible to follow the [iPhoneDriver HOWTO](https://code.google.com/p/selenium/wiki/IPhoneDriver) and also try out the other ios WebDrivers mentioned at the top of that page.

[Documentation to follow later tonight](http://datasift.github.io/storyplayer/environments/remotewebdriver.html).