Auto merge of #1674 - wca:add-freebsd-snapshot, r=alexcrichton

This branch includes a change to dl-snapshot.py that makes it less directly linked to the format of snapshots.txt, and also adds an initial FreeBSD x86_64 cargo snapshot.  The original URL for my snapshot is:

http://people.freebsd.org/~will/public_distfiles/cargo-nightly-x86_64-unknown-freebsd.tar.gz

SHA256: de678e858de5d2923cc206b7aa1cc84ddc67be00748160f66fc9b7325409e7c7