Improved the resetting of the driver

The history is now reset as well (this was already the case in
MinkBrowserKitDriver 1.1 but was changed after that because of a wrong
test code in the Mink driver test)
Added a missing resetting of the form instances
Changed the behavior when there is no request

Instead of returning null for getCurrentUrl, which might break the code
for getCookie and does not respect the expected return type, the method
will throw a DriverException, as this is an invalid usage of the driver
anyway.
Improved the exception message when getting a crawler before visiting
Merge pull request #63 from stof/improve_reset

Improve driver reset