The Go developers decided to move the go.crypto repository to
golang.org/x/crypto, and also to transition from hg to git.  The tip of
tree code.google.com copy of the code is broken due to the import paths
pointing at the new repository.

While the change here is simple (just update the import location), this
affects packagers as it now expects the updated package.  Sorry for the
inconveneince, I blame the Go people.

Exhaustively testing padding combinations is really slow, and was
causing timeouts during the Debian ARM package build process.  Attempt
to improve the situation by:

 * Reusing the client and server keypair for all of the tests, to cut
   runtime down by  ~50%.
 * Splitting the client side and server side tests up, as it appears
   the timeout is per-test case.

If this doesn't fix things, the next thing to try would be to reduce
the actual number of padding lengths tested, but that is a last resort
at the moment.

Instead of "node-id" and "public-key" that are Base16 encoded, use
"cert" which contains the "node-id" and "public-key" in Base64 encoded
form.  This is more compact and cuts the length down by 49 characters.

Write an example client bridge line suitable for use with the running
obfs4 server instance to "obfs4_bridgeline.txt" for the convenience of
bridge operators.