Supports 'H' and '90ms*' CMaps

Fixes #696.
This patch depends on TextDecoder which has just been added since Firefox 18.
In Workers, TextDecoder is unavailable until Firefox 20. So I shimmed it using FileReaderSync.
If neither TextDecoder nor FileReaderSync is not supprted, or if the CMap is not in the supported list, fallback the existing behavior with a warning message.