Added RunOnAddr to listen on custom host:port

Martini relied on global env variable to determine which host:port
address it would listen on. This is inconvenient for cases when more
than one instance is required. RunOnAddr has been added so that a
standard host:port string can be passed.
Merge pull request #295 from mohamedattahri/feature-run-on-addr

Added RunOnAddr to listen on custom host:port