adding Data Encryption Key variant methods.
adding aes decrypt, renaming the DEC methods to DEK (data encryption key)
Merge pull request #3 from jonnolen/addAESDecrypt

Add aes decrypt
Merge branch 'master' of github.com:Shopify/dukpt into feature/ecb-mode

Conflicts:
	lib/dukpt/encryption.rb
Clean up.
Merge.
Fix issue during merge.
Clean up this, again.
Add a test for the crypto mode.