Implement "getchaintips" RPC command to monitor blockchain forks.

Port over https://github.com/chronokings/huntercoin/pull/19 from
Huntercoin:  This implements a new RPC command "getchaintips" that can be
used to find all currently active chain heads.  This is similar to the
-printblocktree startup option, but it can be used without restarting
just via the RPC interface on a running daemon.

qa/rpc-tests/getchaintips.py

@@ -0,0 +1,24 @@
+#!/usr/bin/env python
+# Copyright (c) 2014 The Bitcoin Core developers
+# Distributed under the MIT/X11 software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+# Exercise the getchaintips API.
+
+# Since the test framework does not generate orphan blocks, we can
+# unfortunately not check for them!
+
+from test_framework import BitcoinTestFramework
+from util import assert_equal
+
+class GetChainTipsTest (BitcoinTestFramework):
+
+    def run_test (self, nodes):
+        res = nodes[0].getchaintips ()
+        assert_equal (len (res), 1)
+        res = res[0]
+        assert_equal (res['branchlen'], 0)
+        assert_equal (res['height'], 200)
+
+if __name__ == '__main__':
+    GetChainTipsTest ().main ()