[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)

Yes, I also thought that maybe we shouldnt allow free transactions by the hardcoded priority at all.
On the other hand, smartfee needs some people to send free transactions in order to have a reference.

But this is the wrong pull request for this discussion, here coincontrol just needs to follow CWallet:CreateTransaction.
If you want that to be changed, please create a separate issue for it, as this would require CWallet:CreateTransaction to be changed.