Lower chunk_size_byte to 12 (256KiB)

- In anticipation of streaming decryption
- Firefox 34 does not support chunk_size_byte > 24

256KiB is almost as fast as no chunks (although both of those can be up to
~1.5x slower than optimally using threads for very large message sizes).
The optimal chunk size would be something like:

    max(data.length / navigator.hardwareConcurrency, 128KiB)

But we don't do so currently because

- We don't know the hardwareConcurrency of the decrypting machine
- Smaller chunk sizes are better for streaming decryption

src/config/config.js

@@ -75,7 +75,7 @@ export default {
    * @memberof module:config
    * @property {Integer} aead_chunk_size_byte
    */
-  aead_chunk_size_byte:     46,
+  aead_chunk_size_byte:     12,
   /**
    * {@link https://tools.ietf.org/html/rfc4880#section-3.7.1.3|RFC4880 3.7.1.3}:
    * Iteration Count Byte for S2K (String to Key)