Use POST (instead of GET) request for command data sending

Currently a GET method is used to communicate with a Selenium server. This quickly becomes a problem when amount of JavaScript sent to `getEval` function increases. For example string, that is 4071 symbols long already creates the `Request URI Too Large` error.

In this PR I've changed code to use POST request when there is some data to be sent. In all other cases the GET method is still used.

Related issue: Behat/MinkSeleniumDriver#46