added decrypt fn, and test fn for decrypt and encrypt data.

fixed bug in encrypt, the buffer length of the dest buffer wasn't a
ref, so the len of the buffer was being lost.
adding test file and makefile for test.

 This currently doesn't work :( due to include issues with libbitdht
and libretroshare, I need to fix the make file so that it's possible to
include the auth.cc file from the test dir.