Add support for rabbitmq manager

The web interface for RabbitMQ is something dotCloud used to offer in the old `rabbitmq` service. With a few changes, your recipe can add the same feature.

This also adds a slightly more dynamic calculation of the RabbitMQ high water mark, which, unfortunately must be computed relative to all the RAM on the machine rather than just the RAM reserved for the dotCloud service container. Users should edit the `run` script to set the RAM they've reserved for the service, or possibly update it to read the value from an environment variable (which could be set via `dotcloud env set`)