Improved documentation.
General functions moved outside of the class.
finalized the test_sub and test_sub_value in the test_spectrum.py
Merge remote-tracking branch 'oscar'

Conflicts:
	spectraplotpy/importer.py
Improved test..
added the __rmul__()  in the spectrum.py
Improved tests.
finalized test_mul() and test_mul_value() in test_spectrum.py
Merge branch 'master' of git://github.com/lidiabressan/spectraplotpy into lidiabressan-master
Normalizing the content of the files
Starting the documentations.
Merging from pull.
Adding examples to the README
Merge branch 'docs'
general plotting spectra tool.
Added functionality to plot multiple spectra.
Added comments to plot_spectra.
CVS and Aviv exporters.
Importer corrected and test_importer improved.
added the test_div() in the test_spectrum.py
added plot() and y_error_plot() in the spectrum.py