[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)

Sure, its intentional, we have reduced the minrelayfee from 10000 to 1000 satoshis in 0.9. But this did not make transactions cheaper for the people, because the wallet still paid 10000 (which is good, to ensure everybodys txs get relayed). After the 0.9 release there were even blogs about "How to make your bitcoin transactions cheaper", they told people to start bitcoin with -mintxfee=0.00001000 (1000 satoshis). Back then this was a little dangerous, as your tx may not be relayed, but now its safe. This is just the second step of staging.

So, yes, we should definitely reduce the mintxfee to 1000 satoshis for the 0.10 release,
See #3305.

Bitcoin transactions are super cheap these days as we also removed the rounding, a 192 bytes transaction only costs 192 satoshis. Its actually 1 satoshi per byte.