implement plaintext onion routing

[fkleedorfer]

first, the routing should work without encryption - we just use base64 encoding

[fkleedorfer]

task is half done:

• message is onion-routed, dummy 'encryption' is base64 encoding in CryptoService (onion-crypto)
• the initial payload is currently just a dummy string
• the originator's URI is not yet set
• message is sent through the chain until the exit node is reached
• the exit node is not prepared to handle a http request as payload yet
• the response path is not yet implemented
• the chain nodes do not yet remember the return path for each chain

[fkleedorfer]

closing: the remaining steps are covered by

24 - implement the way back

20 - tunnel an http request through the chain

16 - replace dummy encryption by real encryption