Calling router.map now appends the routes instead of replacing them

I would have added a test but there did not seem to be any route
generation tests.
Added functionality to Router.map to allow it to be called multiple times without the map being overwritten. Allows routes to be added at runtime. One test with multiple cases also added.

Conflicts:

	packages/ember-routing/lib/system/router.js
Merge pull request #2892 from igorT/router_append

Calling router.map now appends the routes instead of replacing them