(CFACT-33) Add dhcp_servers fact for OSX and Linux.

Adding a dhcp_servers fact for OSX and Linux.
Adding logic to discover the primary interface based on routing
information.  We still fall back to first interface with an assigned
address if need be.
(maint) Update example in README to configure log4cxx.

Adding code to example to configure log4cxx.
Without this, users would get a warning from log4cxx.