Updates to installation instructions for Virtual Machine and Docker images

Staged at http://stg-web101.sw.joyent.continuuity.net/cdap/2.5.0/en/getstarted.html
Listed and created sections for the three different download methods.
Updated instructions for Virtual Machine and Docker.
This step is only needed if you don't run on Linux. In MacOS or Windows, boot2docker starts the Docker Linux VM, inside which all of the docker containers will run. ```boot2docker ip``` is also only needed in that case. perhaps you can have a short section about: 

If you are not running on Linux, you need to start the Docker Virtual Machine before you can use containers. For example, on MacOS you type: ```boot2docker start`` followed by ```boot2docker ip``` to determine the Docker VM's address (you will need to use that as the host name when connecting to the Console or making an HTTP request). 

After that section, continue with these instructions: docker pull, docker run, ...