blake2b: register BLAKE2b

Register BLAKE2b-256, BLAKE2b-384 and BLAKE2b-512 if the package is imported.
Change-Id: Iba425a62b42ed07219f7e56ff173ed5ae503704e
Reviewed-on: https://go-review.googlesource.com/36877
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>