Roll version to 0.1.9.
Add TukeyWindow class.
Add FFT.power_of_two? method.
Add Plotter.plot_signals method.
Add flag for Signal#envelope to produce envelope as new Signal object.
Add Signal#remove_frequences and Signal#keep_frequencies.
For Signal class, if method modifies current object, add ! to method name. Many methods have ! and non-! version.
Update changelog and readme for 0.1.9 release.