Working AES-128, -192, -256 decryption

Caveats:
* Doesn't currently check the authentication code
* Won't read zip files which have a data description block without
  the signature. (See notes in code)
* Has no tests yet...
Fixed io seeking bug.
Merge pull request #1 from jphastings/aes-128

Working AES-128, -192, -256 decryption
Merge pull request #3 from githubmo/aes-128

Fixed io seeking bug.
Fixes `Zip::File.open` with encrypted zips.
Don't attempt to set the password on normal decompressors
Fixed bug to allow decompressor's password to be set