Replaced Pongo instances with MongoClient.

 - implemented callback buffering
 - first step to remove Pongo as dependency
 - other improvements in storing options

Fixes #6 

 - [x] Replace Pongo completely
 - [x] Use MongoClient connections
 - [x] Callback buffering
 - [x] Remove Pongo as dependency
 - [ ] Server object cache (`wontfix`)
 - [x] Client object cache