Bring MXNetJS up to date with current MXNet HEAD

[Piyush3dB]

I recently pushed apache/mxnet#5090 to bring the JS amalgamation build up to date with MXNet HEAD. This PR is a continuation of that body of work and brings MXNetJS in sync with the MXNet HEAD.

Specifically:

1. Added script to automatically download models from the MXNet model gallery and prepare JS compatible JSON formats suitable for running with the provided examples.
2. Added the SqueezeNET model example in the repo. This replaces the fastpoor model which is no longer available in the MXNet model gallery.
3. Extended the JS unittest to run more models.
4. Added more documentation to the Readme.
5. Updated mxnet git submodule to pick-up the current HEAD.
6. Updated the compiled JS libraries to the current version.

This PR also resolves issue #11.