Add mock date utility for testing dates and timezone offsets.
Add support for parsing and emitting Date headers, with tests.

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extra : rebase_source : 0e531171a09e5b20d383929fad8dd116765b7a23
extra : amend_source : 1dee407ef192145cd371d5d9ad4ae0cd6ac4ba75
Add Date structured headers.

--HG--
extra : rebase_source : c92ee926333d15e2ad41a39430b71caa8c486b71
Add under-tested Content-Disposition.

--HG--
extra : rebase_source : 3c509a7797f1063aa5df69cf717ebf1ba1cd8411
Import all of the changes from master onto the new jsmime version.

--HG--
extra : rebase_source : 4c2ae1e08db71fc014528df9f15ed4ea3f82b6a9