Integral on `PolyArea` not yet implemented

[mikeingold]

The Meshes.PolyArea geometry type doesn't have a parametrization function defined -- and probably won't ever have one. Once SimpleMesh integrals are implemented (#27), this could probably also be implemented using something like

function integral(f, polyarea::PolyArea)
    mesh::SimpleMesh = discretize(polyarea)
    return integral(f, mesh)
end