Update build for latest NaCl SDK.

Build for ARM. Also tidy up how we find paths. Some (not yet working) stubs for
PNaCl.
Use navigator.webkitPersistentStorage.

It seems the spec has changed yet again, but let's at least move from the
deprecated API to the not-yet-deprecated API. Closes #34.
Use PPAPI's DidChangeFocus hook for focus.

No more need for the JavaScript workaround. Closes #32.