Bug 746052 - Implement a JS-based MIME parser, part 1: add the parser. r=irving sr=Neil

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extra : rebase_source : 62dcb20ac169332185e6dc5934ed0c571067a2a3
Bug 746052 - Implement a JS-based MIME parser, part 2: add parser tests. r=Standard8

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extra : rebase_source : 042352c40fd65dc6e78e2bbfec785b3d5ab38093