Add Closure compiler externs and an eventtype enum for paper-checkbox. This is dependent on the externs for paper-radio-button (https://github.com/Polymer/paper-radio-button/pull/8) and Polymer (https://github.com/Polymer/polymer/pull/769).
Add ignores to the bower.json for externs and event types.
Move externs and eventtype to an externs directory and update bower.json.
Merge pull request #18 from jklein24/externs

Add Closure compiler externs and an eventtype enum for paper-checkbox.