[Qt] Add Smartfee to GUI

By default a minimized label is shown. It updates automatically, if smartfee has been chosen.
![smartfeegui1](https://cloud.githubusercontent.com/assets/2814559/4878732/5f153366-6317-11e4-8733-74bf9d78c892.png)
Smartfee not initialized -> pay minfee
![smartfeegui2](https://cloud.githubusercontent.com/assets/2814559/4878731/5f136a90-6317-11e4-97d8-04f8378c44a4.png)
Smartfee initialized
![smartfeegui4](https://cloud.githubusercontent.com/assets/2814559/4878730/5f117802-6317-11e4-8341-296896b15a5f.png)
![smartfeegui3](https://cloud.githubusercontent.com/assets/2814559/4878733/5f165688-6317-11e4-8b11-cc46555bd0a4.png)
![smartfeegui5](https://cloud.githubusercontent.com/assets/2814559/4878735/5f1c6316-6317-11e4-9794-a6e0a3497b45.png)
![smartfeegui6](https://cloud.githubusercontent.com/assets/2814559/4878734/5f1662c2-6317-11e4-9c79-4048a205e0de.png)
![smartfeegui7](https://cloud.githubusercontent.com/assets/2814559/4878736/5f2b3b2a-6317-11e4-9a4e-92c2c3547942.png)
Fee removed from optionsdialog
![smartfeegui9](https://cloud.githubusercontent.com/assets/2814559/4878737/5f316590-6317-11e4-9725-1f2c13ebdaec.png)

I'm not sure `AllowFree` is a good default here, that's the minimum possible priority to be included in a block, but isn't any indication that it's ever going to happen.