isolate the storage part as a module

splits up `remotestorage-server.js` into a part that deals with actual storage requests (300 lines) and a part that deals with how the portal, webfinger, and oauth are handled on localhost (150 lines).

the actual `starter-kit.js` then launches this together with one webserver for each subdir of the `apps/ directory (75 lines).

the slimmed-down `remotestorage-server.js` should now be easily unit-testable, and for the other two files this should also not be too hard, so i'll add some unit tests before merging this.