Add persisted entity id to JSON-LD payload as @id

[sohkai]

We currently ignore the persisted ids of returned entities, which makes it very difficult for users to save the results on their own backends as well as use other parts of the interface that require an entity's id (e.g. transferring rights).

Although not technically correct, as we don't support GET operations yet (see #14), we can use the @id in the returned JSON-LD to denote the persisted id. Doing so results in a relative URL to the current route, which works for /manifestations and /rights, but will require some special handling for RightsAssignments since its route is /rights/transfer (e.g. its @id will probably look something like ../rights_assignment/<id>).

[TimDaub]

@sohkai If this is blocking you might wanna add this to: bigchaindb/org#22

[sohkai]

@TimDaub actually the PR's available at #16. Transfers are blocked by this, so I treated it as a subtask of its implementation.