Add `registerEntryPoint` to HmrServer
Add `registerEntryPoint` to HmrServer

Summary: This diff changes a few things around so that a diff coming on top of this stack will be smaller. The aim of this change is to add a method `registerEntryPoint` which will allow a client to subscribe to updates for multiple bundles.

Reviewed By: gaearon

Differential Revision: D16131963

fbshipit-source-id: d460d6647b15a711021c7a3a51f52486a1aea535

hoangpham95/react-native,myntra/react-native,javache/react-native,exponent/react-native,arthuralee/react-native,hammerandchisel/react-native,exponent/react-native,hoangpham95/react-native,exponentjs/react-native,javache/react-native,exponent/react-native,javache/react-native,pandiaraj44/react-native,hoangpham95/react-native,myntra/react-native,janicduplessis/react-native,exponent/react-native,exponentjs/react-native,pandiaraj44/react-native,facebook/react-native,exponentjs/react-native,pandiaraj44/react-native,exponent/react-native,janicduplessis/react-native,facebook/react-native,janicduplessis/react-native,hammerandchisel/react-native,myntra/react-native,janicduplessis/react-native,arthuralee/react-native,janicduplessis/react-native,arthuralee/react-native,myntra/react-native,facebook/react-native,exponent/react-native,hoangpham95/react-native,javache/react-native,javache/react-native,facebook/react-native,exponent/react-native,facebook/react-native,hammerandchisel/react-native,janicduplessis/react-native,exponent/react-native,facebook/react-native,hammerandchisel/react-native,hoangpham95/react-native,myntra/react-native,myntra/react-native,myntra/react-native,hammerandchisel/react-native,exponentjs/react-native,hoangpham95/react-native,javache/react-native,exponentjs/react-native,javache/react-native,javache/react-native,myntra/react-native,facebook/react-native,exponentjs/react-native,pandiaraj44/react-native,janicduplessis/react-native,pandiaraj44/react-native,hammerandchisel/react-native,exponentjs/react-native,hoangpham95/react-native,facebook/react-native,pandiaraj44/react-native,hoangpham95/react-native,exponentjs/react-native,arthuralee/react-native,hammerandchisel/react-native,facebook/react-native,javache/react-native,myntra/react-native,pandiaraj44/react-native,arthuralee/react-native,janicduplessis/react-native,hammerandchisel/react-native,pandiaraj44/react-native