0.9.0

Minor release that will update some defaults and interfaces:

• Use np.ndarray instead of np.matrix for all internal matrix representations
• Change default time base from dt=None to dt=0 (PR #431)
• Use use_legacy_defaults to allow execution of code that worked in an earlier release
• Update markov() to use standard convention for time-series data
• Soft drop of Python 2.7 (new features/functions may require Python 3)