[API] new method added IPath.getFileExtension()

Change-Id: I2ae1fbd7fdb04e95cf38735755c08d6e1efdbd4c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1648
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>