allow pass extra settings for portiacrawl close #63

now besides add setting to `slyd/slyd/settings.py`, portiacrawl also allow user adding extra settings from command line. e.g. `slybot/bin/portiacrawl -v slyd/data/projects/new_project/ www.yelp.co.uk -a start_urls=http://www.yelp.co.uk/biz/barrafina-london -s DEPTH_LIMIT=1`
I am not sure if verbose should default to true. Also I would make the help message more accurate, something on the lines of "Prints the command used to run the crawler".