add shell spv testing mode

To try uspv, do ./lnd -spv
The remote node is hardcoded in shell.go.  If you aren't
running a full node on localhost, specify where to connect to.
Nearby nodes will be much faster but random testnet nodes on the
internet should also work.

.gitignore

@@ -37,3 +37,7 @@ test_wal/*
 
 # vim
 **.swp
+
+*.hex
+*.db
+*.bin