Fix compiler warnings

These 2 commits make the build much cleaner. I was seeing warnings for almost every source file until I made these changes, which makes it harder to spot "real" problems.

The warnings were like this:

    In file included from src/zerocoin/Zerocoin.h:53:0,
                     from src/main.h:13,
                     from src/kernel.h:7,
                     from src/kernel.cpp:7:
    src/zerocoin/Coin.h:52:43: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
      const CoinDenomination getDenomination() const;
                                               ^