(#1717) Added BitcoinWrapper::validateaddress() function

Without this patch, the call to $bitcoin->validateaddress() in public/include/admin_checks.php always fails, causing the admin to be informed that their 'cold wallet address is SET and INVLAID'. This happens as long as a cold wallet address is defined, regardless of if the address is valid or not.

The problem can be traced back to public/include/classes/bitcoin.class.php where we define the BitcoinClient::validateaddress() function. This function simply returns true or false depending on the result of a call to parent::validateaddress($coin_address). In BitcoinClient's case, parent is always itcoinWrapper. However, we have never defined BitcoinWrapper::validateaddress().

To resolve this, I simply defined the BitcoinWrapper::validateaddress() function.