Pretty printing

# Pretty printing (for easier debugging and better support for code generation):
    - Finish precedence list to improve parenthesis use in __str__ and __repr__ and ufl2latex.
    - Implement C renderer
    - Rewrite ufl2latex as described in its comments (use pre- and post-handlers)
    - Improve dot-rendering with compact symbols for all node types
      (evt add option to switch between repr style and greek style)
    - Add dot-rendering to graph class: G = Graph(e); print G.dot_format()
    - Add utility "showdot(e)" for interactive experimentation. Need a good way to show images to do this.