InteractiveLegendPlugin added to plugins.py

InteractiveLegendPlugin added to plugin.py and made pep8 compliant.
test_interactive_legend.py added to test_plots

A simple demo of the interactive legend is added to test_plots. It uses
the same code as the legend example, but now in an interactive form.
When running test_plots.py, the legend labels appear to be cut of the
figure. This still needs to be addressed

In addition there are various changes in the interactive legend plugin
itself in order to handle both the lines and markers in case of Line2D
instances. The used solution is based on the discussion in #194.
notebook explanation of the interactive legend

A more detailed illustration of what can be done with the
InteractiveLegendPlugin.
Changes as indicated by Aflaxman

Notebook now imports the legend, rather than having the code in the
notebook.

The two superfluous logs to the console have been removed.

The test code on which there was a comment is a straight copy from
test_legend.py with the legend command replaced by the interactive
legend plugin
Merge pull request #195 from quaquel/InteractiveLegend

Interactive legend