It's not caching all fields - just id fields

Because there is nothing wrong with premature optimization :P

Conflicts:
	mongo/collection.go

SQL: Adding common IDSetters of type int64 and uint64. NoSQL: Adding common IDSetter interface of type ObjectId.

Applying the same style fixes as in #19: Running goimports (https://github.com/bradfitz/goimports) and Fatal(err) instead of Fatalf(err.Error())

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.