Change encryption method to make it compatible with other languages libs

[sametmax]

See #39 for context.

[gebi]

From a security point of view using using aes256/chacha20 instead of using a hybrid scheme with ed25519 is always preferred.

Currently the best recommendation would be to use chacha20poly1305. But as this is quite "new", xsalsa20poly1305 would be the more compatible choice regarding different implementations.
(chacha20poly1305 was also recently proposed as an additional cipher spec for TLS, and is already used between chrome/android and google servers in production).

(copied from #39, fits better here if there already is a dedicated bug for this topic)