Functions passed in virgilio.http are now stringified

To conform to the concept that Virgilio only handles stringifyable content on its bus (to allow it to be used with, for instance, queues), I made a change that stringifties the transform, respond and error functions before submitting over the bus, and parsing afterwards. This has to go allong with virgilio pull request 25:
https://github.com/icemobilelab/virgilio/pull/25