CRAN mirror MD5s validation check

[stephlocke]

Verify a CRAN mirror's package MD5 matches the canonical source's MD5 to ensure the integrity of a download from a specific mirror

[hrbrmstr]

Prbly the best place to keep notes on this. Managed to finally get around to doing the full mirror.

It's ~149GB (I figured it'd be larger)

I'll start gen'ing MD5's soon (work stuff has crept into my day off today so it might be a bit).

[richfitz]

Thanks! I'll probably sit on this for the next week or so and then try and patch up and document the CRAN verification bits