Support non-standard ssh port, doc updates, -p option change

This patch does the following:

- Supports machine definitions of the form ```[user@]host[:port]```
- Changes behavior of ```-p n``` option from "total of n processes" to "additional n worker processes". This has been done to reflect the fact that ```@parallel``` and ```pmap``` both only use workers processes for computation when ```nprocs() > 1```
- doc updates

closes #4298