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^^
Absolutely. I made the same comment two days ago, but sadly github hides all line comments when the file changes, even if the commented line hasn't changed.