Hash blocks and check against their IDs

While the MAC provided by secretbox guarantees both integrity and
confidentiality under the content hash-derived key, holders of this key (i.e.
anyone who can decrypt the block) can also produce ciphertexts under that same
key. To ensure we've received the correct ciphertext, we need to compute its
hash and ensure it matches the block ID.
RIP :(
Make way for Cryptosphere::Block-based primitives

This is all old code using non-NaCl based encryption. It will be placed with
primitives implemented atop Cryptosphere::Block
rbx builds failing :(
Use NaCl as our cryptographic entropy source

One more bit of OpenSSL gone
Move Blake2bXSalsa20Poly1305 under Crypto::Block::Primitive
Add pry to Gemfile
Factor encoding into Cryptosphere::Encoding and add tests!

Moving junk out of the Cryptosphere module seems like a good idea
Bundle the latest everything
Rip off the OpenSSL band-aid

Best line of this commit:
-require 'openssl'

This commit completely removes the OpenSSL dependency, replacing it with a new
initial head system based on Cryptosphere::WriteHeads and
Cryptosphere::ReadHeads for storing pointers to URIs in the system.

Cryptosphere::Identity has been deleted for now with no suitable replacement.

There should no longer be any shitty crypto in the Cryptosphere. It should be
gone now. Please go nuts looking for problems ;)
Making progress
We now use BLAKE2b instead of HKDF
Move heads under the Cryptosphere::Head module
Use plural names for directories

This is more "Rails"-y right? LOL
Delete unused legacy constant

LOL 2048 bits. Big by ECC standards. Small for McEliece!
Add one spec, with a lot more pending

Add a spec for giving WriteHeads a URI with the wrong scheme

Add more pending specs.
YARD documentation
New logo (with new hidden message)

This is the third Cryptosphere logo puzzle. There is a new hidden message
encrypted in the binary numbers that line the logo image's background.

The previous logo's message was encrypted with a Vigenère cipher. This one uses
a more difficult cipher.

For more information on the logo ciphers, see:

https://github.com/cryptosphere/cryptosphere/wiki/Logo-Ciphers
Fix rbnacl dependency
Link to documentation from README