Add alert_text selenium API

Pulled these two selenium API calls into nightwatch:
https://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/alert_text

FYI setAlertValue works on firefox, but there's a bug in the chromedriver [1]

[1] https://code.google.com/p/chromedriver/issues/detail?id=831&sort=-id&colspec=ID%20Status%20Pri%20Owner%20Summary