Installation Quick Start Section (CDAP-695)

• Add an installation quick-start section. (Fixes https://issues.cask.co/browse/CDAP-695)
Staged: http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-2.6.0_doc_updates/en/admin-manual/installation/quick-start.html
• Add to installation index page.
Staged: http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-2.6.0_doc_updates/en/admin-manual/index.html
• Re-arrange installation guide to flow better.
• Improve progress messages from build script.
• Add "Removing an Application" to the getting-started section.

Staged at: http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-2.6.0_doc_updates/en/admin-manual/index.html