Improved pretty printing

[Martin Sandve Alnæs]

From launchpad. Various forms of pretty printing can benefit from some updating and improvements, for easier debugging and better support for code generation:

- Lift C renderer from uflacs
- Lift latex renderer from uflacs, improve ipython notebook support
- Implement better latex renderer for forms, improve ipython notebook support
- Rewrite str renderer with algorithms from uflacs
- Improve dot-rendering with compact symbols for all node types
  (evt add option to switch between repr style and greek style)
- Lift improved graph class from uflacs, add dot-rendering: G = Graph(e); print G.dot_format()
- Add utility "showdot(e)" for interactive experimentation. Need a good way to show images to do this. Maybe with ipython notebook support?

[Prof Garth Wells]

Closing for now, see https://bitbucket.org/fenics-project/ufl/pull-requests/105.