Add support for assigning a Function with linear combinations of Functions, 
which lives in the same FunctionSpace
  -- Allows syntax like
     u.assign(u0+u1/4-3*u2)
  -- Only works in Python
Small change in ChangLog
Merge with trunk
Tweaked error message and allow:

   scalar(scalar*func-scalar*func)
Fixes for linear combination of TensorCoefficients in assignment operator
merge with trunk
Make it possible to use Constant as a scalar value in the Function.assign method.
Merge with trunk
Add FunctionAXPY which serves as an intermediate for linear combinations 
of Function, which lives in a the same FunctionSpace. Functions can be 
assigned a FunctionAXPY object.
Move a function specific typemap
Add ButcherScheme and RKSolver classes. The former will be populated with 
a generated list of forms and stage solutions. The latter takes a ButcherScheme 
and steps it.
Add generation of butcherscheme. Fill step method with content
Added more ButcherSchemes and test for vector form
Merge with trunk
Added fixes for new Assembler interface
Add basic demo for the runge-kutta solver.
  -- It is based on the advection-diffusion problem
  -- It now runs much slower than original demo, but some optimizations
      might fix that
Add PointIntegral sketch. Need to merge with trunk to get added 
dofmap functionality before finishing
Merge with trunk
Add work on PointIntegralSolver:
  -- Added logic of extracting ufc::point_integrals from UFC
Merge work with trunk