Bitcoin dividends

Change the dividend address displayed in the Receive tab to a Bitcoin address.

Change the Shares...Export Peercoin keys title and dialog by substituting Peercoin for Bitcoin.

Maintain the ability to export to a full Bitcoin client, but also include an option to display the Bitcoin private keys (paired with the public key) in a window whose contents can be selected and copied to the clipboard.

Once an issue is assigned, it appears it cannot be unassigned. My intent is to leave this unassigned so that either sigmike or eraspunk can choose to complete it.

Solution proposed in pull request #19.

Comment from Jordan:

I had hoped the user would see their private keys in a window. It looks like the RPC dumps them, but we want them displayed on screen just using menu navigation

Solution proposed in pull request #23.