Allow specification of IP address for service

The current address_for method requires that a service pass in the
interface that it wishes to bind to.  It then looks up the first
IP address on the interface.  This does not work when there are
multiple IP addresses assigned to the interface.  This change adds
the address_for_service method that allows a service to specify the
IP address to bind to.  If no IP address is specified then this
method will fall back to calling the address_for method.
Merge pull request #51 from cdearborn/pull-req-master-88686cbab23956b8d02c9dc282d28541ec72e87b

Added ability to specify IP address [1/1]
Fixed comment
Merge pull request #52 from cdearborn/pull-req-master-38ee16941eb0be971a5d6b562c475aeabea13458

Fixed comment [1/1]