Fix on Ubuntu 14.04

Go isn't able to access elements within unions within structs (hopefully that makes sense...), so we need to add a few wrappers to get this working on Ubuntu 14.04.

Let me know if there's a better way to do this. I'm not a fan of adding overhead like this :-/.

@xaprb @gkristic @juphoff @ElPeque 

Fixes https://github.com/VividCortex/golibpcap/issues/10.
Bit of a nitpick, but this comment is out of date now, isn't it? Seeing as this PR adds more wrappers to fix issues with Ubuntu 14.04.