Remove JSON Spirit wrapper, remove JSON Spirit leftovers

- implement find_value() function for UniValue
- replace all Array/Value/Object types with UniValues, remove JSON Spirit to UniValue wrapper
- remove JSON Spirit sources

src/test/sighash_tests.cpp

@@ -16,9 +16,9 @@
 #include <iostream>
 
 #include <boost/test/unit_test.hpp>
-#include "json_spirit_wrapper.h"
 
-using namespace json_spirit;
+#include "univalue/univalue.h"
+
 extern UniValue read_json(const std::string& jsondata);
 
 // Old script.cpp SignatureHash function