Made fig optional in Spectrum.plot() and plotter_helper.plot_spectra().

It checks if fig is none and assigns matplotlib.pyplot to fig if no other value is set.
TODO: Is fig really the right name here?
plot_spectra returns the figure/axis used for plotting.
Merge branch 'master' of https://github.com/ajasja/spectraplotpy into plotting

Conflicts:
	spectraplotpy/spectrum.py
Renamed fig to axes where appropriate in Spectrum().

This fixes SpectralGroup/spectraplotpy#35.
Also renamed fig to axes in plotter_helper.plot_spectra().
The axes in now passed in the **kwargs to Spectrum.plot() and errorbars().
Merge pull request #39 from ajasja/plotting

Thanks