Discussed changes for private-config and the slightly adjusted documentation

Hi Julien.

As discuessed yesterday, I've added a short part about the private-config.js thingy to the documentation and changed the configuration process.

The process now 'mixes' the configurations - the main configuration, as discribed in the db/setup/config.js is now used as a template and everything in the private-config.js will override or add values. 
To make it quite easy to do, I've added _node.extend_ as a dependency. This is a adaption of the jQuery mix-in function implementation 'extend'.

Cheers,
Holger