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.
As I mentioned, using Docker on MacOS and Windows is not that straight-forward. I think the Docker image is really for people who develop on Linux. We should target this section for Linux developers.