Give md5sums on the download page

[kmsquire]

At the least, this would be useful in finding possible sources of download corruption (cf. #4980)

Perhaps the main julia packages, should be signed (and public keys distributed), since these are most likely to be made by trusted members of the core team.

[ViralBShah]

There is also an issue open for signing binaries on Mac and Windows. #2381

[quinnj]

ping @staticfloat @ihnorton

[staticfloat]

Mac, Windows and Ubuntu builds are now all signed, which means that they're already getting checksummed by your OS. They won't run if they don't pass the checksum verification (at least not without a big scary error). Those are really the only binaries we distribute, so I think we're good here.