Add set in table creation

[annismckenzie]

TL;DR: I needed a way to tell gocqltable to handle a slice in the model struct definition as a set in CQL and not as a list.

Not really happy with the amount of code needed to get this in here. Along the way, I fixed a few of go vet's concerns regarding certain code constructs, safely exposed the struct information map to the outside world (now usable in your project using gocqltable) and added a way to query indexed map/list/set values (or keys) via CQL's CONTAINS clause. There's a few basic tests added in there as well so maybe there's parts of this you want to pull into upstream.

I'll be using Go 1.6's vendor feature to embed my fork in the current project (forks were a real pain before – maybe we should do this for the dependencies of gocqltable as well?) so I'm not in a rush to get this merged.

Thanks for the library!