BUG: fix collections in legend
a line html tooltip

Modeled on the existing html tooltip, but now specific for Line2d
objects.
pep8 changes
Minor readme fixes.

- Fixed linked to notebook examples.
- Deleted last bullet under features, since additional interactivity is implemented.
Merge pull request #189 from ajasja/readme-fix

Minor readme fixes.
Merge pull request #183 from quaquel/LineHTMLTooltip

a html tooltip for Line2d objects
Merge pull request #182 from jakevdp/legend-collection

BUG: fix collections in legend
Fix README reference to notebooks directory
Merge pull request #192 from dloss/fix-readme-examples

Fix README reference to notebooks directory
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.
Remove installed mplexporter directory before installing if necessary
Removed rm and rsync with shutil.rmtree and shutil.copytree,
Error checking and moved shutil methods into seperate subroutine for clarity
fixed bug to support python 3
added check for existing submodule source directory
notebook explanation of the interactive legend

A more detailed illustration of what can be done with the
InteractiveLegendPlugin.
Merge pull request #180 from jakevdp/clean-mplexporter-install

Remove installed mplexporter directory before installing if necessary
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