Replaced "Bitcoin" with "Peercoin" in string literals

Replaced "Bitcoin", "bitcoind", "Bitcoin-Qt", and "BTC" with their Peercoin equivalents. This was not automated so as to avoid breaking anything; this is purely a cosmetic change. No actual functionality was altered. In addition, it isn't 100% conclusive; there are still a few strings that contain "Bitcoin" so as to avoid confusion when changing the functionality for Peercoin.