v0.4.0: New metrics, normalized Taylor plot, other improvements and bugfixes

• Add normalized Taylor plot
• Add aggregate_observations argument to taylor plot
• Add new metrics: max_error and willmott's Index of Agreement
• Improve TrackObservation item argument: now takes both int and string
• Improve TrackObservation input check, now require input to be specified if more than 3 items
• Bugfix: normalize NSE metric with obs
• Bugfix: remove class attributes that caused problems with multiple objects
• Add obs_item to fmskill.compare()
• Improved getitem to handle negative indexes and raise correct errors
• Bugfix: fmskill.compare() select item
• Improved metric documentation