Extract REST from braid-corda

An app that doesn't have anything to do with Corda, should be able to use braid to host both a JSON-RPC and a REST end point. For REST it should get all the benefits of the REST package.

Currently, REST (and Swagger) are in braid-corda, where they're intermingled with Corda dependency.

This should be refactored so that REST is extracted into braid-core (independent from Corda), from where it can be included in braid-standalone-server