Add user management ability

This PR adds the following:
- user_management recipe, which will require auth on mongo config
- user resource and provider
- default users attributes
- user_management integration tests

Should take care of #225
this won't work with multiple mongos on the same box (I know nothing does, but I'd like to avoid having to change this later). Same goes for the `retrieve_db`'s access of `node[:mongodb][:host]`.

Perhaps a connection object like in [opscode's `database` cookbook](https://github.com/opscode-cookbooks/database)?