LaTeX representation for Brian objects / ipython notebook integration

Add a LaTeX representation to core brian objects (in particular units and equations) that works nicely with the ipython notebook.

- [x] LaTeX for equations
- [x] LaTeX for units (e.g. ``$\omega$`` for ``ohm``)
- [x] LaTeX for scales (e.g. ``$\mu$`` for ``u``)
- [x] LaTeX for explicit state updater descriptions
- [x] LaTeX for NeuronGroup/Synapses
- [ ] anything else?