added unit test for convesion from operator to vector to operator
reverting 229f588c5bcd574f104120bff841c1705be07cbe because we need to remain compatible with how spre/spost do the tensor products (and they do a transpose before the tensor product)
minor typo fix
added support for superoperators instead of operators in rhs_generate
support for superoperators instead of operators in propagator
fix state space bug in propagator for liouvillians
Merge pull request #125 from jrjohansson/extend-propagator

better internal support for working with sparse matrices when calculating propagators
added option new keyword argument to plot_winger and plot_wigner_fock_distribution to choose 3d surface plot instead of contour plot
Merge pull request #128 from jrjohansson/extend-plot_winger

extended plot_wigner
increase the tolerance threshold in qutip/tests/test_superop_reps because 1e-12 occationally results in false errors
increase tolerance in expm test in qutip/tests/test_qobj because the previous value 1e-12 has repeatedly been reported to result in false errors on 32 bit systems
explicitly set B operator to superrep choi in choi_to_chi to avoid warning about multiplying superoperators with different represenantations
pep8 compliance
Merge pull request #129 from jrjohansson/fix-super_ops_reps

Fix super ops reps
resolved merge conflict