OptionalFeature class added
support for optional deps in SpikeBrowser, PlotFeatures, PlotSpikes
tests for base.OptionalFeature
corrected cluster_beans example
implemented membership test operation for FeatureBroker
docstrings for RequiredFeature and OptionalFeature
style fix for components.py
fix for style fix for components.py
further style fix
Merge pull request #88 from btel/optional_features

Optional features implementation (closes #82)