Otherwise exceptions raised on commit are not caught by the api exception view deriver.

refactor(exceptions): define custom api exception, a view deriver to raise them and view exception to display them

Fixes #32 and cubicweb/cubicweb#680

Closes #30.

Which is more pythonic.

Reverts to the base pyramid_jwt implementation but with a custom serializer which simply returns the given value. Needed as the JWT token is already base64 encoded.
This fixes authentication issues using the cubicweb_web login forms.

Errors are logged using the info level

This allows us to change how cookies are created

related: #2