Standalone app: accept skinny.port as a sys prop

It was only looking for an environment variable, but it should also look
for a system property.
Add SKINNY_PORT as a supported env variable

Because env vars with dots in them are quite painful to work with
Merge pull request #80 from cb372/bug/skinny-port-sysprop

Accept skinny.port as a sys prop; Add SKINNY_PORT as a supported env var