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 is slightly misleading. The CDAP Console does not replace it. It will say something similar to "Connect to dashboard at http://6f0162922c37:9999". The 6f0162922c37 is the virtual host name of the docker container. The user needs to replace that with the IP address obtained by ```boot2docker ip```.