Improve Grunt support.

* remove clean since concat already overwrites the dist files
* improve JSHint support
* update all devDependencies
* simplify package.json and add `test` script
Update .travis.yml.

* remove node 0.8
* add grunt-cli
Update demo code to the latest online version.

Also fix the validation errors and clean up JS code.
Demo: tab to spaces.
Rename jquery.easing.min.js to reflect the fact its not minimized.
Fix JSHint issues and made code style consistent.
README.md: add devDependency badge.