Add support for MongoDB Server

A basic lens and test to support the configuration of MongoDB

Added infrastructure components for MongoDB

Included references to MongoDBServer to handle the auto testing of
lenses on build & mention of lens in for autodoc generation.

fixes hercules-team/augeas/#25

Improved documentation on MongoDBServer filter

Added improved reference and usage information for the lens

Mistyped port path in MongoDBServer lens usage

The port component was missing from the set command

fixes herclues/team-augeas/#25