Change impact layer classes

Hi @timlinux, this PR will fix some issues:
- Volcano Impact layer is not correctly clasified https://github.com/AIFDR/inasafe/issues/702
- Flood / population impact layer results https://github.com/AIFDR/inasafe/issues/637
- No failing tests on safe and safe_qgis package related to this changes.
- ```create_classes``` is being used by ITBFatalityFunction (haven't tried the case), FloodEvacuationFunction (see the 4th case below), FloodEvacuationFunctionVectorHazard (see the 3rd case), VolcanoBuildingImpact (see 1st and 2nd case), and VolcanoPolygonHazardPopulation (haven't tried this yet)

The idea on creating classes is to create the first white style (FFFFFF) with the non affected. And will populate the second minimum value to the max value to the rest classes.

Here are some comparisons using the same case:
1. Volcano Building Point IF.
The top is using this PR, the bottom is using the old classification.
![cmp volcano merapi](https://cloud.githubusercontent.com/assets/992519/2935675/782e4020-d822-11e3-8149-88367e707291.png)

2. Volcano Building Polygon/KRB IF.
  - This case is for Merapi KRB. The top is using this PR, the bottom is using the old classification.
![cmp - merapi krb](https://cloud.githubusercontent.com/assets/992519/2935674/66add5ea-d822-11e3-99f0-286ed32d0d18.png)
  - This case is from this issue https://github.com/AIFDR/inasafe/issues/702. The top is using this PR, the bottom is using the old classification. 
![cmp - atl](https://cloud.githubusercontent.com/assets/992519/2935664/b9955fb8-d821-11e3-8a82-8518a1f1d21d.png)


3. Flood Polygon on Population Raster
As you can see that this PR will try to find the 2nd minimum value of the affected.
![flood polygon population](https://cloud.githubusercontent.com/assets/992519/2935683/b521edf6-d822-11e3-9a85-d47512b0959b.png)

4. Flood Raster on Population Raster
This case will fix https://github.com/AIFDR/inasafe/issues/637
![flood](https://cloud.githubusercontent.com/assets/992519/2935684/c599913e-d822-11e3-957d-7d57027ce977.png)