Separated stdlib imports from other imports.

Looks like #13 conflicts with some mongod versions (https://travis-ci.org/upper/db/jobs/24479509), rolling back to old solution to test it a bit more.

The old Collection.UpsertId() trick to get the _id value of the latest inserted item stopped to work. This workaround allocates a new _id and then updates the item.

Fixing db.Fields for MongoDB, related: https://groups.google.com/forum/?fromgroups=#!topic/gosexy/3QcW40qZYfk