JSONRPC.py added.

This new class is the lowest level so far. It's the responsible for handling
HTTP requests and return the JSON response.

It can ask for all or specifics permissions, allowing a fine control of XBMC
remote calls.
A ping method checks for connectivity. And version method return the XMBC
system's version.

The documentation is coming, I promise. For now, the code is very well written.
System.py updated.

The last commit adds JSONRPC class, who now handles the HTTP post requests.
System class now inherit JSONRPC and some error treatment was added.

A little fix was added to JSONRPC  class in order to allow multiples requests using the
same object.

Again, documentation is coming.