add more spectral functions and add new spectrum types to specgram im mlab, and cleanup 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

even more axes tests

even even more axes

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