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.
"the .ova file" is rather confusing here. When you go to the download page, there is no mention there of an ova file. All there is a "Download Standalone VM". Perhaps you can rephrase this to: 

- Download the CDAP Standalone Virtual Machine at http://cask.co/downloads.
- Import the downloaded .ova file into VirtualBox or VMWare Player
- ...