Support destination URLs with query strings

[JKingsnorth]

Currently URLs with query strings are not supported as final destinations, eg:
/user/login?destination=page%3Freset%3D1

The module fails on the raven_signature_check on line 205, producing a 'raven authentication failure' error message.

I think this is because we're trying to url decode the data, which is causing a mismatch because of the special characters it generates, which is broken by the base64_decode.

Removing the url decode from the raven_signature_check seems to fix the issue. PR incoming.

[JKingsnorth]

#47