Add description for package.json

Make the description on [npm](https://www.npmjs.org/package/ember-validations) not "[![Build Status](https://secure.travis-ci.org/dockyard/ember-validations.png?branch=master)](http://travis-ci.org/dockyard/ember-validations)"
Merge pull request #179 from kimagure/patch-1

Add description for package.json