Generating correct Key & IV. Not integrated yet.

I've set up a decryption decompressor which sucessfully decrypts a test zip
to the deflated bytes however the following things aren't implemented:

* Setting the password
* Detection of the strength of the AES decryption used (128, 192, 256)
* Deflation of the decrypted bytes so you can get at your zipped data
* Re-initialising the decrpter (`#prepare_aes`) for each file
* Checking the authentication code of the entry (the encrypted data's CRC)
* Encryption (I'm not planning to do this)