Allow passing in a custom user agent.

Custom user agent can be passed using the ``--user-agent`` parameter e.g. 

``phantomjs phantomas.js --url=http://github.com --verbose --user-agent='Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3'``