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

Notes:
- No failing tests on safe and safe_qgis package related to this changes.
- ```create_classes``` is 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``` (see the 5th case)
- 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. Most people will more likely assume that the 'white' zone is a not affected zone.

Here are some comparisons of the old and this PR's classification:

1. Volcano Building Point IF.
The top impact layer 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. As you can see that using old classification, 585, 604, 338 will be classified to the white class.
![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. Using the old classification, even though there are 1980 buildings affected, it will be classified to the white class, and all other classes are in the same range.
![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. The top impact layer is using this PR, the bottom is using the old classification.
![flood](https://cloud.githubusercontent.com/assets/992519/2935684/c599913e-d822-11e3-957d-7d57027ce977.png)

5. Volcano Polygon/KRB on Population Raster
This case generates the same classification since the minimum value is not 0 or fall within the special case I handle on the new classification.
![workspace 1_005](https://cloud.githubusercontent.com/assets/992519/2941020/d69c8efe-d995-11e3-9775-642fb0bf5e95.png)