Minor style improvements

components.jsx.js
1. Use React.addons.classSet
2. Don't call _.filter when we want all
3. Use _.countBy rahter than _.reduce

stores.js
1. Factor out duplicated code for finding item by key into method
   _itemForKey
Cleaner way to count elements, using _.filter
Bump version of react-router to 0.7.0
Merge pull request #3 from justin808/minor-style-suggestions

Minor style improvements