BUG: some browsers don't recognize dasharray=10,0; change this to none
dasharray: use 'none' rather than null
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.
Fix README reference to notebooks directory