remove an option to pass kwargs to System.Hamiltonian

doc/source/images/ab_ring.py.diff

doc/source/images/quantum_well.py.diff

doc/source/tutorial/ab_ring.py

doc/source/tutorial/closed_system.py

doc/source/tutorial/quantum_well.py

doc/source/tutorial/tutorial2.rst

doc/source/whatsnew/0.3.rst

kwant/_system.pyx

kwant/builder.py

kwant/physics/dispersion.py

kwant/physics/tests/test_noise.py

kwant/plotter.py

kwant/solvers/common.py

kwant/solvers/tests/_test_sparse.py

kwant/system.py

kwant/tests/test_system.py