Add host and container info to DockerInfo API #1091

This PR addresses issue #1091 and adds the following information to the DockerInfo API endpoint and the `docker info` command:

- Number of running containers
- Memory usage of running containers
- Max memory usage of all containers (combined memlimits)
- Number of Logical CPUs on the server
- 1, 5, and 15 minute CPU Averages of the server
- Free RAM on the server
- Total RAM on the server

It's probably rough around the edges, so I would really appreciate any feedback.