Added support for custom headers in PhantomJS driver.

I'd like to test web applications supporting multiple languages.
To do so, additional request headers like 'Accept-Language: en-US,en;q=0.8,ja;q=0.6' is required to be set in HTTP request message.

This patch allows PhantomJS driver to send custom headers to the web apps, by specifying 'custom_headers' parameter in dict format when creating Browser instance.

Also, please note that it requires phantomjs >=1.9.2.