File en-/decryption with Streams

This adds en-/decryption which can handle big files by using Streams.
Additionally a test file is added to test this function (only ~100 Bytes).
#150 is closed by this.
First #146 and #161 have to be merged, then I need to rebase^^
Ah, sry, now that I see it, ivOS is a stream that gets initialized directly in this method, so there shouldn't be any error thrown to the outside anyway.
We still have to make sure that those bytes are actually written, otherwise.. well, log it and return null. Throwing an IOException probably will confuse the user as he/she thinks there is some real IO action going on inside.