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 browser should be started outside of docker. Do you think that is obvious? I guess it is... because in docker you only have a shell prompt. I guess no need to mention it.