Add port option to duckpan server to allow port specification

A dev asked on the community platform if it was possible to specify a different port for the DuckPAN Server. It wasn't, but I figured that was a reasonable request so I've added in that functionality,

cc// @jagtalon @russellholt @mwmiller @jdorweiler 

You can now specify a port using `duckpan server --port 1234` or ` -p 1234` or `--port=1234`

Picture of it working:
![2014-08-21_16h12_51](https://cloud.githubusercontent.com/assets/873785/4002740/bb25ff6e-296f-11e4-847a-bd42ca85b937.png)