Fix CDAP startup scripts to correctly check for the current version

Fix for https://issues.cask.co/browse/CDAP-26:
- Moves version info from amazon s3 to docs.cask.co. (http://docs.cask.co/cdap/version)
- Fixes an error in cdap.sh that prevented the script from checking the version.
- Enhances the information returned to show both current and new versions.
also, maybe a separate bug, but we need to check if curl exists