Add Support for BLAKE2

[espeed]

BLAKE2 overview: https://blake2.net/

[xsc]

@espeed Again, thanks for the request. There seems to be a Java implementation of BLAKE2b at alphazero/Blake2b but I'm not sure I want to increase pandect's footprint with another dependency. Any opinions on whether it's a good idea to add another crypto provider besides the Java default one and BouncyCastle?

[zooko]

Bouncy Castle 1.54, released 2015-12-29, comes with BLAKE2! ☺ https://www.bouncycastle.org/releasenotes.html

[xsc]

The BLAKE2b variants provided by BouncyCastle have been released in

[pandect "0.6.0"]

I also adjusted the way BouncyCastle algorithms are used, removing most of the BC-specific code, so we should have it easier to add new algorithms in future.