Implement point_integral

Implement code generation for new class point_integral including the functions

   /// Tabulate the tensor for the contribution from the local vertex
    virtual void tabulate_tensor(double* A,
                                 const double * const * w,
                                 const cell& c,
                                 std::size_t vertex) const = 0;

    /// Tabulate the tensor for the contributions from a set of points
    virtual void tabulate_tensor(double* A,
                                 const double * const * w,
                                 const cell& c,
                                 std::size_t num_points,
                                 const double * const * points) const = 0;

The first version is completely analogous to the standard cell/interior facet/exterior facet case. [And is in place as of Feb 2013.]

The second version requires the same infrastructure as tabulate_tensor with
given arbitrary quadrature points for the cell/interior facet/exterior facet integrals.
In particular, the follow is needed (and can be reused for all of these four integral types)

* Implement map_to_reference_cell
* Implement evaluate_basis_reference
* Use these for the point_integral::tabulate_tensor.

The implementation of the latter case would be natural to do for the four integrals simulataneously.