Somewhat of a work-in-progress (in that MIME is a large spec), but this is
functional and enough for discussion and/or code review.

In addition to the unit tests, I've tested with curl and Chrome with
a variety of test files, making sure the digests of files are unaltered
when read via a multipart Part.

R=rsc, adg, dsymonds1, agl1
CC=golang-dev
https://golang.org/cl/1681049