Automate the build of the shipped client-side frameworks

I have slightly modified the procedure used to build the Engine.IO library.
Now it doesn't use the `engine.io-client` dev dependency but it clones the `engine.io-client` repo and builds the library there.
In the same way i have added a utility to build the SockJS library.

There may be some code duplication in the two `update.sh`.

Finally i have added a small orchestration script in the `package.json` which allow us to automatically update the client-side frameworks running:
```shell
npm run update
```