Add support for parsing and emitting Date headers, with tests.

--HG--
extra : rebase_source : 0e531171a09e5b20d383929fad8dd116765b7a23
extra : amend_source : 7b132e7c62d3443eaf97f009c53a9b7e9a7c0001
Add Date structured headers.

--HG--
extra : rebase_source : 314094d70b127a47428713d68316732cd9f3ffb3
Add under-tested Content-Disposition.

--HG--
extra : rebase_source : 639fc36eb7e4dcea3209af708756b40749ab6de8
extra : amend_source : a105ad513c678d56ef0305206b328d4149c29368
Import all of the changes from master onto the new jsmime version.

--HG--
extra : rebase_source : 13c10eb22b0f81681382a14f9527442092dbe4aa