plotter_helper.plot_spectra now sets the datasets name as label.
Implemented plotter_heler.minmax_normalize

Normalize a spectrum in place by dividing it by the max(abs(y)).
After normalization all the y values are between [-1 and 1].
Renamed spec to spectrum in plotter_helper.minmax_normalize
Merge pull request #60 from ajasja/minmax

plotter_helper.minmax_normalize