Make the type of normal vectors a Tensor.

Make the type of normal vectors a Tensor.

As discussed in #496, normal vectors are currently returned by FEValues
as Point<spacedim> objects, but since they are differential vectors, the
correct data type should be Tensor<1,dim>. This patch implements the solution
discussed in #496 by
* changing the return type of FEValues::normal_vector()
* deprecating FEValues::get_normal_vectors()
* introducing FEValues::get_all_normal_vectors() that returns a vector
  of tensors.

The old get_normal_vectors() function was deprecated. In order to
make it work, I also had to change the return type from a reference
to a straight array. This array may be bound to a reference in
many places in user codes, but the style we have used everywhere
is to make these reference variables very localized in scope and
so I don't foresee any problems.