Fix Input Message Overwrite Bug

[cwacek]

Thanks for putting this project together - it's really useful.

This fixes unexpected behavior I discovered while using block encryption. The input array was always overwritten to be identical to the output. I created an optional parameter which controls this behavior (and disables it by default).

I've also updated the README a bit to clarify some of the differences between how the CryptoJS API works and how this library's API works.