automatically download prettify files

This would remove the need to manually install the prettify files.

In https://github.com/ibm-js/jsdoc-amddcl/issues/40 @cjolif suggested to use grunt to do that but I think using the npm postinstall hook is more adapted to solve this issue.
I guess you're right. From the node doc I see that console.warn and console.error are the same so I'll go with console.error everywhere.