Update README.md

Fixed broken links.
Use ExitCommand in cdap-cli
(CDAP-24) Update CDAP to release version of Twill (0.4.0-incubating)
- Also remove the need of apache snapshot repo.
Merge pull request #634 from caskdata/feature/fix-cli-exit

Use ExitCommand in cdap-cli
Merge pull request #636 from caskdata/feature/twill-0.4.0-release

(CDAP-24) Update CDAP to release version of Twill (0.4.0-incubating)
Merge dev
Store ACL list instead of boolean flag