Inconsistency between help message and actual behavior of --risk option

```
$ python sqlmap.py --version
1.0-dev-99c1cc9
```

From

```
$ python sqlmap.py -h | grep risk
    --risk=RISK         Risk of tests to perform (0-3, default 1)
```

risk can be 0, but

```
$ python sqlmap.py -u 'http://example.com' --risk=0
[CRITICAL] value for option '--risk' must be an integer value from range [1, 3]
```
