Add address to Peercoin wallet (issue #2)

Here's an implementation for issue #2.

To communicate with peercoin we need many config data (rpc host, credentials, etc). I chose to load the peercoin config for that, in a separate `mapPeercoinArgs`. Its default path for this config file is the same as peercoin's, and it can be overridden with `-peercoinconf`.

I had to duplicate some code. I chose to duplicate whole functions instead of adding a flags or something, to avoid altering too much the original code and allow easy merges. I'm not sure what's best for future maintenance.

I also changed the default ports and config files. I took the first things that came to my mind so you may want to change them.

Right now the new address imported from peercoin (in `CWallet::GenerateNewKey`) is only printed in the log.
If the RPC command fails an exception is thrown and the qt client exits. I'm not sure how we should handle that.