protocol: Enable private offers

Users can send private offers that will only be seen by one address.

To send a private offer:
 1. recipient creates a new address to receive offers on
 2. recipient sends the pubkey for the newly created address to the offerer
 3. offerer imports the recipient's pubkey
 4. offerer sends a new offer to the recipients key instead of the public network

Nodes will ignore offers sent on keys other than the network key or keys created for offer-receiving.

basicswap/templates/offer_new_1.html

@@ -8,6 +8,12 @@
 <form method="post">
 
 <table>
+<tr><td>Send To</td><td><select name="addr_to">
+<option{% if data.addr_to=="-1" %} selected{% endif %} value="-1">-- Public Network --</option>
+{% for a in addrs_to %}
+<option{% if data.addr_to==a %} selected{% endif %} value="{{ a }}">{{ a }}</option>
+{% endfor %}
+</select></td></tr>
 <tr><td>Send From Address</td><td><select name="addr_from">
 {% for a in addrs %}
 <option{% if data.addr_from==a %} selected{% endif %} value="{{ a }}">{{ a }}</option>