Support "web element identifier" in updated W3C standard used by Firefox's GeckoDriver

[lkende]

Running the current jWebdriver against Selenium 3+ and GeckoDriver, elements are not correctly identified.

Older implementation expected by jWebDriver when a call to retrieve an element. The body response to the wire protocol will use: "value":[{"ELEMENT":"1"}]

Newer implementation used by GeckoDriver and presumably by future webdriver implementation, the body response contains the element value as: "value":[{"element-6066-11e4-a52e-4f735466cecf":"4ba8d8d3-5f4d-4007-99e9-30fac0d76f3b"}]

You can see the hard-coded attribute element-6066-11e4-a52e-4f735466cecf that should be referenced in the W3C docs for element

To support both standards, I am submitting a pull request with the updated correction.