rename models to lib

[jcscottiii]

Feeling inspired by https://peter.bourgon.org/go-best-practices-2016/#repository-structure
To make it clear in terms which packages should be imported by other projects, we should have a lib folder. I think models is essentially that.

After we do that, we need to go through the code and see what's exported vs private inside lib
The interfaces should be exported. The concrete structs should be private.