Mojo JS Bindings: Shell wrapper class

Restored the support for the JS Shell wrapper class that had previously been part of the "mojo.js" module.

Modified the JS wget example to use the Shell class.

Added one small test where the JS app sends a response to a client. Refactored the JS apptest test code.

BUG=
R=aa@chromium.org

Review URL: https://codereview.chromium.org/771353002