Add swap/

Add a binary crate `swap` that implements two nodes (Alice and Bob). With this
applied we can start up a node for each role and do:

- Bob: Requests current amounts using BTC is input
- Alice: Responds with amounts
- Bob: (mock) get user input to Ok the amounts

... continue with swap (TODO)

swap/src/cli.rs

@@ -0,0 +1,14 @@
+#[derive(structopt::StructOpt, Debug)]
+pub struct Options {
+    /// Run the swap as Alice.
+    #[structopt(long = "as-alice")]
+    pub as_alice: bool,
+
+    /// Run the swap as Bob and try to swap this many XMR (in piconero).
+    #[structopt(long = "picos")]
+    pub piconeros: Option<u64>,
+
+    /// Run the swap as Bob and try to swap this many BTC (in satoshi).
+    #[structopt(long = "sats")]
+    pub satoshis: Option<u64>,
+}