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.
I am not a big fan of how this list is written. We can group it into two bullet lists: one for the VM configuration, one for the software that is configured. Also reorder a little, starting with prereqs, to CDAP itself, to development env.

has is configured with the recommended settings for CDAP:
- 4 GB of RAM 
- Ubuntu Desktop Linux 
- 10 GB of disk space
It also has all the software pre-installed that you will need:
- Java JDK 7 and Node.js are both installed.
- Maven is installed and configured to work for CDAP.
- The Standalone CDAP is installed under /Software/... and will automatically start when the virtual machine starts. 
- Both IntelliJ and Eclipse IDE are installed and will start when the virtual machine starts.
- The Firefox web browser starts when the machine starts. Its default home page is the CDAP Console (http://localhost:9999).
No password is required to enter the virtual machine; however, should you need to install or remove software, the admin user and password are both ``cdap``.