This includes fix for #3 and some clean up based on advancement in netaddr codebase

Hi @tubaman,

I'm filling in your lack of time to fix issue #3 by addressing search in django admin.
Also, I'd like to know why you chose to implement db type as BigInteger.
If there's no good reason behind it I'd like to change it to a char field as this enables to search MACs' using partial bits of a MAC (for example last 4 chars).

First 2 commits are clean up based on the netaddr code base. Those hacks which I removed are no more necessary with updated netaddr library (seems like it's safe to use netaddr from almost a year back).

Custom db type is not so urgent that I'll see to it later.