Generalize handling WHERE IN clauses

[annismckenzie]

This cleans up the changes I made in #3. It pushes clauses for WHERE col IN (?) statements into the where clause handler, thereby drops the string value constraint and then lets gocql fill in the values correctly interpolated and escaped.

If you want I can get started on a few basic tests – however, I've been using WHERE IN extensively in our codebase and the changes are sufficiently easy to understand.