Expose GEOSPointOnSurface; add UI elements to fTools Polygon Centroids ...

... plugin to allow requesting points to be inside polygon if centroid falls outside.

This commit make the `GEOSPointOnSurface` function available in Python inside QGIS, in the same fashion that `GEOSGetCentroid` is currently available (i.e. as a function of a `qgsgeometry`). It also adds a checkbox to the fTools Polygon Centroids plugin allowing the user to choose between returning the centroids or returning a point inside the polygon if the centroid falls outside the polygon.

Screenshots:
[Default behaviour](https://f.cloud.github.com/assets/2399255/1186347/ba1f4bca-22f4-11e3-8d7d-61e483d20c1a.png)
[With "Always return points inside polygons" selected](https://f.cloud.github.com/assets/2399255/1186346/ba0955ea-22f4-11e3-937a-843125a1c81b.png)