[fix] wrong implementation of Meta-Operation  and
[improv] all meta-operation
[improv] add change_collection, drop_database, drop_collection
[improv]
- Index: add, drop and some helper
- Better implementation for drop_collection and drop_database
[doc] generate documentation
[fix] drop index bug
[fix] lwt implementation for parallelised thread
[fix] documentation
[improv] count cmd
version 0.67.0
Merge pull request #7 from msimon/master

Mongo.ml improvement