Fixed #52 -- Add URL encoding to otpauth URL

Google Authenticator gave an error when scanning the code using the
previous format. I added url quoting as well as the optional `issuer`
URL parameter.

Modified otpauth label format. Changed from `Site Name (username)` to
`Site Name:username`, which appears to be the most common format, although
both would probably be valid.

Modified test cases to reflect the changes made in `QRGeneratorView`
and `get_otpauth_url` (otpauth label and issuer parameter)
Standardized oauth URL geneator argument names

The revised method uses the names as defined in the specs, so it becomes
easier to identify them. Extra tests have been added to verify that the
generated URLs are correct.
Fire signal when user is verified

A signal will be fired when a user is verified. This would allow extra
customization like notifying the user when a backup device was used. See
the added documentation for an example implementation.