Fix error in Firefox < 4.
Update underscore tests and vendor again.
Simplify `_.isEmpty`.
Ensure `pluck` always used `property` and `findWhere` and `where` always use `matches`.
Avoid `!=` in `createCallback`.
Use `_.noop` more in test/test.js.
Add tests for `_.pluck`, `_.findWhere`, and `_.where` for their `key` or `source` arguments.
Rebuild dist.