add test and documentation for "draft" capability

doc/zmq_has.txt

@@ -27,6 +27,7 @@ defined:
 * norm - the library supports the norm:// protocol
 * curve - the library supports the CURVE security mechanism
 * gssapi - the library supports the GSSAPI security mechanism
+* draft - the library is build with the draft api
 
 When this method is provided, the zmq.h header file will define
 ZMQ_HAS_CAPABILITIES.

tests/test_capabilities.cpp

@@ -73,5 +73,10 @@ int main (void)
     assert (!zmq_has("vmci"));
 #endif
 
+#if defined (ZMQ_BUILD_DRAFT_API)
+    assert (zmq_has("draft"));
+#else
+    assert (!zmq_has("draft"));
+#endif
     return 0;
 }