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