Make Firefox add-on typings compatible with TypeScript 1.1

Fix several errors reported by the current version of TS 1.1
from github.com/Microsoft/TypeScript
Work around missing IDBKeyRange static interface in TS 1.0

The static side of IDBKeyRange's interface is missing from TS 1.0's
lib.d.ts. Import the current definition from TS 1.1's lib.dom.d.ts file.