Merge pull request #11 from mborne/master

Building generator using a Straight_skeleton to generate roof
Fix triangulation of a MultiPoint
Fix constness
Add a C API
Fix convexhull on empty geometry
Fix polygon construction from linestring with double points
Add a type when returning empty geometries (mimick GEOS)
Fix results of an intersection: returned geometries are now filtered
Fix parsing of EMPTY in WKT
Move C API into a 'capi' directory
Fix convexhull when calling on a geometry collection of empty geometries