Add element event APIs

[alechenninger]

TODO: details

[alechenninger]

For implementing, look into:

1. WebDriver's execute async which has a javascript callback (so can hook into browser's event loop)
2. Proxy all web traffic, add an event listener via javascript that makes an ajax call, and intercept that call (look at Browsermob proxy)

[alechenninger]

Pending darcy-framework/darcy-ui#39