Make types stricter for JSCompiler.
Update Moment.js to 2.2.1.

Combines lang files and core Moment, as done in Moment.js itself.
Add externs for chrome.app.getDetails missing from Closure contrib externs.
Allow specifying only one of text, color or title as needed.

Explicitly allowing undefined satisfies JSCompiler when only one of the
three is passed along to a method.
Handle HTTP errors in feed fetches.
Pass null to the callback when zero events are available.

Also initialize startTime and endTime to blank strings instead of
undefined.