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^^
No there is no reason, I changed it a few times and obviously the last change was 2048 to 10(24)48..
I thought a size of ~1KB is a good size, but I don't know how it behaves on mobile devices, so tell me what you think :)