Provides the process running on the given port. This retrieves the results
from our cache, so it only works if we've already issued a query() request
for it and gotten results.

:param int port: port number to look up

:returns: **Process** using the given port

:raises:
  * :class:`nyx.tracker.UnresolvedResult` if the application is still
    being determined
  * :class:`nyx.tracker.UnknownApplication` if the we tried to resolve
    the application but it couldn't be determined
)