Removed the wrapping of driver exceptions into an element exception

Refs #467
Added a driver test for the screenshot method
Made some phpdoc more specific about types instead of using mixed
Merge pull request #468 from stof/exception_wrapping

Removed the wrapping of driver exceptions into an element exception
Added an assertion on the image using GD
Added tests for executeScript

Closes #457
Merge pull request #469 from stof/execute_script

Added tests for executeScript
Merge pull request #470 from stof/test_screenshot

Added a driver test for the screenshot method
Use single-line javascript in testWindowMaximize
The `testWaitReturnValue` test was checking element visibility, which failed for Headless drivers
Merge pull request #472 from aik099/waitReturnValue-test-headless-ready-fix

The "testWaitReturnValue" don't check visibility
Mouse tests fix. Focus/blur events now tests on "input" element and not "div".
Merge pull request #474 from aik099/mouse-events-tests-fix

Mouse tests fix
Removed unneeded hook in the mouse event tests

PhantomJS does not seem to need sleeping anymore in these tests when using webdriver
Adds "Element::isValid" method for checking element presence in DOM
Merge pull request #480 from aik099/element-validity-check-feat

Adds "Element::isValid" method for checking element presence in DOM
Added a bunch of safeguard in tests to avoid fatal errors on failure

If the elements used in the tests are not found, most cases are now
reporting a failure instead of triggering a fatal error by calling a
method on null.
Tests to verify, that window manager moves the mouse
Merge pull request #485 from aik099/native-events-on-mouse-actions-fix

Tests to verify, that window manager moves the mouse
Added testing on PHP 5.6 on Travis