major rewrite to expose (most of) the guts.

the code isn't as pretty to look at, but it will be easier to work
with in the debugger and to really tweak in the options. this is
just the first part of this effort - got tired halfway through.
did you know a 1-point polygon makes a good dot?

and why was i using a path instead of a polyline?
this simplifies things considerably!
factor out the common logic in hilighting and erasing.

i gave getIntersectionList a try (as a much faster replacement for the
current "pick" method, which has to do a lot of messing around with
stroke-width and pointer-events attributes), but it is incorrectly
implemented in webkit (it just uses bounding boxes). sigh.
moved the core dom element into its own class: Doodles.

the name isn't the best, given the other names. but it essentially is the
ModelView part, and what's left in Doodle is now the Controller part.
hope i remember to fix the names so they make more sense.