Set default fee to nMinTxFee and show fee warnings in optionsdialog

- set default fee to CTransaction::nMinTxFee instead of zero, for both bitcoin-qt and bitcoind. For bitcoin-qt only new installations are affected
  while for bitcoind, everbody who has not set -paytxfee is affected
- add default value to --help output for -paytxfee
- qt-optionsdialog: change fee description 
- qt-optionsdialog: add fee warnings
- qt-optionsdialog: minor design improvements: added borders and aligned the database and verification threads controls

Before:
![defaultfee1](https://f.cloud.github.com/assets/2814559/2118889/776743b4-912b-11e3-9c50-c0cb99d6af0e.png)

After:
![defaultfee20](https://f.cloud.github.com/assets/2814559/2118890/7dea0320-912b-11e3-8b2a-7f10effbf33c.png)

Warning if fee >= 5 * minFee. Useful for people who still have the old 50000 satoshis value.
Also useful for people confusing BTC and mBTC...
![defaultfee21](https://f.cloud.github.com/assets/2814559/2118891/8a8dc666-912b-11e3-9de4-1edf1ee92e98.png)

Warning if fee = nonsense value. 0 < fee < minFee
![defaultfee22](https://f.cloud.github.com/assets/2814559/2118893/ae9b588e-912b-11e3-9362-f943e26c2f09.png)

Warning if fee = zero
![defaultfee23](https://f.cloud.github.com/assets/2814559/2118894/b7343aba-912b-11e3-906a-0c9d2f197a87.png)