Add V8 debugging support

This adds a `startDebugAgent` method to the V8Js PHP object which enables V8's debug agent if called.  This allows to debug the JavaScript code executed by V8Js using the Google Chrome Developer Tools Eclipse plugin, available from https://code.google.com/p/chromedevtools/wiki/HowToInstall

That allows for break points, variable inspection, single stepping, etc. ... and pushing back modified code to V8 :-)

Currently debugging is limited to one (main) isolate

Configurable auto-breaking is supported by executeString function.  That way it either waits once (first executeString call) for the debugger to get connected or always, i.e. on every executeString call