Support the Stellar memo convention.

[jedmccaleb]

https://github.com/stellar/stellar-protocol/issues/28

Steps that need to happen when you handle an incoming message:

1. you receive a tx
2. Do you already have a preimage of the memo?
3. no -> Fetch the preimage
   • Look up the home domain of the source account
     • Fetch the stellar.toml
   • or, Look for a "msg_server" key on the account
   • ask the MESSAGE_SERVER for the preimage of the tx memo
4. Check signature on any signed blocks.
5. Are there blocks that are encrypted?
   • Do we have the key?
   • yes -> decrypt
   • no -> ignore

after processing, the sdk should for each operation provide:

• note
• user info
• routing info

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We need to think a bit how we are going to implement this in the various sdk's @nullstyle