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

even more axes tests

even even more axes

add test images for new spectrum plots
increase performance of spectrum-related functions
add demo for new spectral plot types
added information regarding new spectrum functions to CHANGELOG, whats_new, and api_changes