Add ability to search for keys using WKD protocol

If a search pattern that looks like an email address is found
an additional query using Web Key Directory will be performed.

Implements basic flow described in "Key Discovery" [0] I-D.
Querying SRV records is not supported.

Fixes partially #2270.

[0]: https://tools.ietf.org/html/draft-koch-openpgp-webkey-service-05#section-3.1

OpenKeychain/src/test/java/org/sufficientlysecure/keychain/util/ZBase32Test.java

@@ -0,0 +1,14 @@
+package org.sufficientlysecure.keychain.util;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class ZBase32Test {
+
+    @Test
+    public void encode() {
+        assertEquals("yyyoryar", ZBase32.encode(new byte[]{0, 1, 2, 3, 4}));
+    }
+
+}