finalized the test_sub and test_sub_value in the test_spectrum.py
added the __rmul__()  in the spectrum.py
finalized test_mul() and test_mul_value() in test_spectrum.py
added the test_div() in the test_spectrum.py
added plot() and y_error_plot() in the spectrum.py
Merge branch 'master' of https://github.com/odarbelaeze/spectraplotpy
Merge branch 'master' of git://github.com/ariamania/spectraplotpy into ariamania-master
Merge branch 'ariamania-master'
create the exceptions file
added LengthError exception in spectrum.py
imported the custom_exceptions in __init__.py
Merge branch 'master' of https://github.com/odarbelaeze/spectraplotpy
Test plotter_helper implemented.
Merge branch 'master' of https://github.com/odarbelaeze/spectraplotpy

Conflicts:
	spectraplotpy/__init__.py
Exceptions of the spectraplotpy library defined.