Removes out bitcoin mention in favor for zcash

Bitcoin Core => Zcash
bitcoin address => Zcash address
bitcoinaddress => zcashaddress

Closes #1756

src/key.cpp

@@ -221,7 +221,7 @@ bool CKey::VerifyPubKey(const CPubKey& pubkey) const {
         return false;
     }
     unsigned char rnd[8];
-    std::string str = "Bitcoin key verification\n";
+    std::string str = "Zcash key verification\n";
     GetRandBytes(rnd, sizeof(rnd));
     uint256 hash;
     CHash256().Write((unsigned char*)str.data(), str.size()).Write(rnd, sizeof(rnd)).Finalize(hash.begin());