add more spectral functions and add new spectrum types to specgram im mlab, and clean up window and detrend functions
add mlab tests for new spectrum types, window and detrend functions, and greatly expand existing tests
add more spectral plots and add new spectrum types to specgram in _axes
add tests for new spectrum types in _axes
add CleanupTestCase, a unittest.TestCase with cleanup
refactor mlab spectrum, window, and detrend functions to improve modularity, testability, and performance
implement tests for refactored mlab spectrum, windowing, and detrend functions, with 100% coverage
add demo for new spectral plot types
added information regarding new spectrum functions to CHANGELOG, whats_new, and api_changes