Working to bring the NetFlow codec to a higher milestone status

The NetFlow codec is pretty awesome, but the code could use some refactoring. Since it didn't have any tests, I figured that the first place to start was to add some tests that document its current behavior before making any changes.

The hand-coded fixtures are mostly verified against real NetFlow traffic from a Cisco Catalyst layer-3 switch, except that I took a guess at what some of the "complex" data would look like because I'm not collecting that kind of stuff in my environment today.

I'm planning to add some more details, but wanted to get this first set of passing tests out there in case there's more interest in the community.