using custom revel.TypeBinder for bson.ObjectId in form fields

With this revel can parse the .Id field back into the model (and you can just use .Hex in the template to get the string presentation)

I guess the custom Binder should be moved to revmgo so users dont need to care about it but i wanted to discuss it first and not open pull requests on two repos.

ps: this is the first time i played around with reflect so this might be horribly wrong - ill post this PR to the revel mailing list to get some more eyes on it.