Feature to specify custom location for static assets

Hi Raphael, 

Sometimes its useful to let application serve static assets from outside of project directory. Could be as a first to step to let front end server like nginx/cherokee to serve them.

Added property in application.static.asset.basedir in app conf file, which will trigger ninja to serve them from that location.

Added a documentation around this as well in static_assets.

Please let me know your thoughts.
Rgds
Sojin