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.
Another thing to mention is that when you run ```boot2docker start```, it prints a message on the screen similar to:
{noformat}
To connect the Docker client to the Docker daemon, please set:
    export DOCKER_HOST=tcp://192.168.59.103:2375
{noformat}
It is important to actually run this export command. Otherwise the subsequent docker commands will fail (because they can't tell how to connect to the docker VM).