Onion data packets can now be handled in onion_client.

oniondata_registerhandler(...) can be used to register different packet
types to handle them and send_onion_data(...) can be used to send these
packets.
Some packet handling code added to onion_client.
onion_client almost done.
Wrote random_path function.

Added onion_client to the build system.
describe configuration of libsodium path

after failing the installation with described error, I needed some time to realize that there might be a problem with the custom path of libsodium installed by homebrew, needing manual configuration. Hope this saves some time for somebody.
Fixed spelling.