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^^
Not sure if that's too nit-picking, but this method should return null for all exception cases (`or null if authentication tag validation failed or another error occured`) - how should the user notice an error otherwise?