Use local binding instead of dynamic global for collection. Fixes #68

Removes dynamic global `*query-collection*` and instead uses a local binding inside `with-collection`. In addition, binds `~coll` locally to avoid evaluating it more than once in the expansion of the macro.

The full test suite still passes (with the exception of a class version mismatch against Clojure 1.4 with something in mongo.conversion, which failed before I made any changes).