Add Curvature correction term to sst turbulence model

Curvature correction term described in paper
http://iopscience.iop.org/1742-6596/572/1/012056
There's thread about this pull request for OpenFOAM-2.1.x version: https://github.com/Unsupported-Contrib-OpenFOAM/Unsupported-Contrib-OpenFOAM-2.1.x/pull/7