Add script for checking gitian builder signatures.

This script WILL CHECK:

    * That all the gitian builder signatures, in a single directory, made on a
      sha256sums.txt file for a TBB-3.x, can be verified.
    * That the particular sha256sum for a specifix TBB tarball matches the
      appropriate one in the sha256sums.txt file.
    * That the signature made directly on the TBB tarball can be verified.

You still need to download the TBB tarball, the sha256sums.txt file, the
appropriate keys for the gitian builders, and the signature for the TBB
tarball.

This script will probably NOT WORK on OSX, because OSX is a piece of fucking
shit and requires doing some bullshit with tempfiles:

    temp=$(mktemp -t sha)
    grep "$tbb_tarfile" "$sums" > "$temp"
    shasum -a 256 -c "$temp"
    rm -f "$temp"