Reimpl buttonset using reactive template

Doesnt quite work b/c nested views ('each') in reactive is not
quite there.
Use rivets instead of reactive

Note shim for rivets-react adapter, and non-jQuery binders
simpler template (div > a instead of div > ul > li > a)
implement alternate buttonset behaviors (unselectable, multiple)
button select example in test/index
vocab
Events now pass button obj, not slug and label

Update Readme, test/index.html
version bump
remove package.json
acknowledge source in license section
Add ButtonSet#button accessor
Add ButtonSetView#button for accessing button el's

Add data-button-slug attribute to template
ButtonSetView save model ref
Add Button#dropdown property, styling for dropdown caret
Add ButtonSetView#dropdown for integrating dropdown menu into buttonset

add dropdown case to test/index.html, add ericgj/dropdown as dev dependency
Readme fix
Readme add dropdown example
Readme fix