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