Fixed bug by which c.port('<id>', <port>) would fail because when port not a string

e.g. 
c.port('e087bc23f8d5', 80)

would fail

and 
c.port('e087bc23f8d5', '80') would work. I now inserted str() so it should always work.
Merge pull request #17 from dhrp/master

Fixed bug by which c.port('<id>', <port>) would fail