Ability to configure WebDriver timeouts

Fixes #76

There's currently no way of setting the desired session timeouts before the webdriver session is initialised, you have to retrieve the session after it's started and apply them.

This change would allow Behat/MinkExtension to read these timeout settings from a config file or something similar without a code change.

Usage:

    $driver->setTimeouts( [ 'implicit'=>1000, 'page'=>4000, 'script' => 500 ]);