Providing Tasks

Hey,
i've added the [ability to run tasks](https://github.com/joernroeder/sandcastle#providing-tasks) – basically a two-way communincation channel – to the sandboxed script. 
This fixes my issue https://github.com/bcoe/sandcastle/issues/34 as I can can communicate with the script and send file chunks down the wire whenever the script requests them.

I think this another cool feature which enhances the power of SandCastle! What do you think?

Cheers!