Fix IV usage

Per jbtule's comment here:

https://github.com/jbubriski/Encryptamajig/commit/c47aa782f9396488800591b5ccbeb415a5e7ca3c#src-encryptamajig-encryptamajig-aesencryptamajig-cs-P173

We should initialize the IV with a separate key derivation function than the key, and prepend the IV to the encrypted output.

Then we need to fix the decryption method to pull the IV from the front of the data stream.
