adding the ability to add admin and other users to mongodb through recip...

...e, readme updates, kitchen integration tests
adds
- user_management recipe, which will require auth on mongo config
- user resource
- user provider
- users attributes
- user_management integration tests
Might be clearer if these were `node['mongodb']...` since that is how they would be accessed outside of the default attributes files. Updating or using the attributes would be done through the node object.