Added backward-compatible implementation for the `$element->value()` method

Added backward-compatible implementation for the `$element->value()` method that was introduced in selenium v2.42.0

See:

* https://code.google.com/p/selenium/source/browse/java/CHANGELOG (search for `getValue` keyword)
* https://code.google.com/p/selenium/source/detail?r=953007b48e83f90450f3e41b11ec31e2928f1605

Steps to reproduce the issue:

    vendor/bin/phpunit --filter testTypingViaTheKeyboard Tests/Selenium2TestCaseTest.php

This test would emit the `GET /session/:sessionId/element/:id/value` request that isn't the part of JsonWire protocol. So the selenium v2.42.0 (and higher) return `HTTP 500` response for the request.

For further use it's supposed one will use `$element->attribute('value')`