Add a BSONInt64 type

In python 2.x PyMongo decodes BSON int64 to the python long type. Python 3 only has an int type so BSON int64 is decoded to int. This is a lossy conversion. There are several use cases for storing a low value integer as an int64 in MongoDB, but round tripping those value with python 3 loses information.

PyMongo's bson module should include a BSONInt64 class (I'm open to other names). It should inherit from long in python 2 and int in python 3. The bson codec will always encode it to BSON int64, regardless of value.