(#1725) Fix cold wallet check FP and added testnet detection

Without this patch, admin.php checks if it can connect to the wallet service. Regardless of if that check passes or fails, it then checks if the cold wallet address is valid. If the can_connect() test failed, the validateaddress() check will also fail, even if the address is not invalid. To fix this, the validateaddress() check should be nested inside the 'if can_connect()' call's else-block.

Additionally, this patch checks to see if the wallet service is running as a testnet. While running as a testnet is perfectly acceptable when testing, the user should be warned. A lot of users following the quickstart guide miss this (see IRC).

A function was added to the Bitcoin class to detect when we are running in a test net. A check was added to admin.php, and the existing can_connect and validateaddress() calls were restructured to solve these issues.