rewrite plotter for matplotlib

doc/source/images/1-quantum_wire.py.diff

doc/source/images/2-ab_ring.py.diff

doc/source/images/2-quantum_well.py.diff

doc/source/images/2-spin_orbit.py.diff

doc/source/images/3-band_structure.py.diff

doc/source/images/3-closed_system.py.diff

doc/source/images/4-graphene.py.diff

doc/source/images/5-superconductor_band_structure.py.diff

doc/source/images/5-superconductor_transport.py.diff

doc/source/images/html.py

doc/source/images/latex.py

doc/source/reference/kwant.plotter.rst

doc/source/tutorial/1-quantum_wire.py

doc/source/tutorial/4-graphene.py

doc/source/tutorial/tutorial4.rst

doc/source/whatsnew/0.2.rst

kwant/plotter.py

kwant/tests/test_plotter.py