fix and extend CBitcoinExtKeyBase template

- fix Decode call (req. only one param)
- add constructor for base58c->CExtKey

Author: jonasschnelli

src/base58.h

@@ -146,14 +146,18 @@ template<typename K, int Size, CChainParams::Base58Type Type> class CBitcoinExtK
 
     K GetKey() {
         K ret;
-        ret.Decode(&vchData[0], &vchData[Size]);
+        ret.Decode(&vchData[0]);
         return ret;
     }
 
     CBitcoinExtKeyBase(const K &key) {
         SetKey(key);
     }
 
+    CBitcoinExtKeyBase(const std::string& strBase58c) {
+        SetString(strBase58c.c_str(), Params().Base58Prefix(Type).size());
+    }
+
     CBitcoinExtKeyBase() {}
 };