MongoHub issues with connecting to Config servers.

MongoHub has issues reading values from mongo config servers because it uses bson_type for iterating and evaluating through the result set, while config servers can return $minKey and $maxKey for some fields and it will crash because of missing enum's.

This patch replaces bson_type for BSONType from bsontypes.h which contains the values and implements is't own bsonIteratorNext which is used instead of the native bson_iterator_next which also has no support for such fields.

Maybe it's not the most beautiful solution, but I have not seen any support for MinKey/MaxKey in the C driver.