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^^
Shouldn't all kinds of variables in unittest classes be either constants (`final`) or otherwise be declared within the methods?
I just want to syntactically ensure that there's no way of hiding execution states due to specific orders of executed methods.