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