added the api to execute native phantomjs code (ported from java webdriver)

this commit adds the api to execute native javascript code in phantomjs from phantomjs-driver,
there are same little refactorings to clean the code but it's the same of the java webdriver.
the googlecode issue url is https://code.google.com/p/selenium/issues/detail?id=6949.

note: in the class OpenQA.Selenium.Remote.CommandInfoRepository there's also a little performance update inside the implementation of the property Instance to use the lock only the first time