Implement JS flipper client

Summary:
Standardize WS implementation for JS environments.

Why do we need a separate server implementation for browsers?
Browser targets cannot authenticate via the default certificate exchange flow. We need a dedicated client for them that works over an insecure channel (without the cert exchange).

Major changes:
1. Renamed `flipper-js-client-sdk` to `js-flipper` for consistency with `react-native-flipper`
2. Updated `js-flipper` implementation to match our other existing clients

Documentation will be updated in a separate subsequent PR.

https://fb.quip.com/2mboA0xbgoxl

Reviewed By: mweststrate

Differential Revision: D31688105

fbshipit-source-id: 418aa80e0fd86361c089cf54b0d44a8b4f748efa

react-native/react-native-flipper/index.d.ts

@@ -38,7 +38,7 @@ declare namespace Flipper {
     /**
      * Returns true if the plugin is meant to be run in background too, otherwise it returns false.
      */
-    runInBackground(): boolean;
+    runInBackground?(): boolean;
   }
 
   export interface FlipperResponder {