Add the ability to provide a struct to data handlers which passes data through before returning the message

[jpwilliams]

Currently, to get more structured data out of a message, we need to do something like:

type IncomingData struct {
	GivenId  string `bson:"id"`
	TargetId string `bson:"target"`
}

data := new(IncomingData)
b, _ := bson.Marshal(event.Data)
bson.Unmarshal(b, &data)

If we could get this into adding data handlers to do this automatically, that'd be awesome, seeing as it'll be a very common task.

Google's github.com/google/jsonapi would be a good one to go for, though need to figure out how it works.

Currently, as above, most endpoints used for our systems utilise gopkg.in/mgo.v2/bson.

[jpwilliams]

Maybe we could provide a set remit.Parser function that returns an EventDataHandler?
We'd have to add something like a Custom interface{} on Events that we can tap into, then:

package remit

import (
	"fmt"

	"gopkg.in/mgo.v2/bson"
)

func Parser(s interface{}) EventDataHandler {
	return func(e Event) {
		data := new(s)
		b, err := bson.Unmarshal(e.Data)
		if err != nil {
			e.Failure <- err
			return
		}
		bson.Unmarshal(b, &data)
		e.Custom = data
		e.Next <- true
	}
}

This means you could use it like so:

endpoint := remitSession.createEndpoint("math.sum")
endpoint.OnData(remit.Parser(IncomingData), addNumbers)