Add Documentation support to the JS command

Ideally I'd like to add some more bits to this, but let's get it in to start with.

Longer term though:
* symlinking the jsDocDir to a sub-directory in the public web root
* Support for --server
* MDK caching of the external.data file (see lib/yui/common/yuidoc.json) from http://yuilibrary.com/yui/docs/api/data.json - this is the bit which makes it really slow to run at present for us. It's fine within Europe :|