Added a test for multiline XPath in form handling

This test was initially written in the SahiDriver testsuite.
Added a test setting the user agent as request header

This test is coming from the ZombieDriver testsuite, which requires a
different code for setting the user agent than other headers.
Added another test for event triggering on input change

This test was initially added for Selenium2Driver only.
Merge pull request #537 from stof/driver_tests

Driver tests moved upstream