Allow subclassing of WebDriver class

[gggeek]

Currently the WebDriver class is declared final.

This is a pity, as the Selenium2Driver has a method setWebDriver which is typehinted against it.
This means that it is not easy for me to subclass either.
I propose that

• either we remove the 'final' keyword from WebDriver
• or we introduce an interface WebDriverInterface (all it needs is the session method) which can be used by 3rd parties for typehinting...

[gggeek]

ps: see minkphp/MinkSelenium2Driver#231

[aik099]

In your particular case what is missing is ability to specify extra curl options for every curl call to WebDriver using JSONWireProtocol.

This topic was already discussed before (and you commented in there), but no viable solution was implemented. The @robocoder said, that ability to change curl options is an edge case and I should be injecting my own curl service to make it work. That's too much of an overhead really, just to set a proxy (what you needed) and curl timeout (what I needed).

See #53, #51

[robocoder]

FYI I also removed the final keyword from the WebDriver class.