Add sample address to NetworkStyles

Label: GUI
GUI application displays sample address in the Pay To input box. It displays main net address even when running on testnet:
![screen shot 2014-12-15 at 20 34 49](https://cloud.githubusercontent.com/assets/6848764/5443077/fc86e9de-849e-11e4-87bb-40dc9dd398b8.png)
This PR adds sampleAddress to the NetworkStyles - it allows for different address for every network.

What address should I use for testnet and regtest?