Create zero balance trust lines with auth flag (RIPD-1003):

This allows a TrustSet transaction to create a trust line
if the only thing being changed is setting the tfSetfAuth
flag.

src/ripple/protocol/impl/Feature.cpp

@@ -47,5 +47,6 @@ feature (const char* name)
 
 uint256 const featureMultiSign = feature("MultiSign");
 uint256 const featureSusPay = feature("SusPay");
+uint256 const featureTrustSetAuth = feature("TrustSetAuth");
 
 } // ripple