change email header parsing. MASSIVE SPEEDUP!

change email header parsing. MASSIVE SPEEDUP!

restructure the email header parsing code to improve speed and to
improve API. MBox::read_header now returns a hash containing all
headers, with keys downcased but nothing else.

update Message#parse_header to accept this new format.

update unit tests to reflect new format.

The exciting news is that changes in how the parsing works have resulted
in a 480% speedup in message parsing speed (634 message/s vs 132
messages/s on my machine), across ALL source types. hell yes.