Provide OS kernel version in system.os.

This is available via the new property called `release`. A very simple
example to demonstrate it:

  var system = require('system');
  console.log('Kernel release', system.os.release);

which will print (on OS X 10.7.5 Lion):

  Kernel release 11.4.2

https://github.com/ariya/phantomjs/issues/12587