Implement compileToBrowserify and bootBrowserApp

Hide `compile` and `execute` and provide a better API for browserified
applications:

 - `boot.compileToBrowserify(options, bundler)` calls `compile` under
   the hood and adds all instructions and scripts to the bundler.

 - `bootBrowserApp(app)` is exported by loopback-boot when the module
   is loaded in a browser, the function loads the instructions as
   bundled by `compileToBrowserify`.

This new API hides all implementation details from the user and makes
it easy to add loopback-boot to any build script.