Document padding

[slxcc]

Reading the source of cipherModes.js and doing a few tests, AES-ECB and AES-CBC seems to use PKCS7 as default padding.
In my opinion, it would be a welcome addition to the documentation since any encrypt or decrypt operation from a third party using another implementation may require those parameters to build the appropriate cipher instance.

[dlongley]

Yes, you're right, this should be added to the documentation.

[jacek213]

Please add this information to documentation, then it would be clear without going through the source code.

[jacek213]

I've made a PR for this: #461

[edgepirate]

@jacek213 @dlongley is it possible to use other padding schemes (ex: pkcs5 for AES-ECB)