Allow model remoting after the explorer is initiated

/to @ritch or @bajtos 

The PR fixes the issue that models remoted after the explorer is initiated are not visible.

It depends on https://github.com/strongloop/loopback/pull/402
I believe it should be easy to filter routes for a given class only - see the code in `addRouteToAPISpec` which is looking up the class name and shared class using the route metadata.

Once we have a function "return the sub-array of routes that are for the given class", this hack will be no longer necessary.