Add Peercoin public address to Peershare wallet (issue #3)

The peercoin addresses are stored in memory in `mapPeercoinAddress` on the wallet. On the disk they're saved in `wallet.dat` with the key `peercoin`.

I also added the RPC command `getpeercoinaddresses` to list the peercoin addresses of your keys.

Note that if you already have a `wallet.dat` the keypool is full of keys without a peercoin address associated. So the first 100 keys you'll create won't have a peercoin address.