Adds support for the PhantomJS browser

Selenium now has native support for the PhantomJS driver "ghostdriver" (webdriver.PhantomJS).  This pull request adds the appropriate method to _browsermanagemnet as well as a unittest.

This change allows Robotframework-selenium2library users to use phantomjs (installed separately) as a browser type.  e.g.:

```
Open Browser    http://google.com    phantomjs
```
PhantomJS can be installed from http://phantomjs.org/download.html