added a bit of documentation and renamed "polys" to "shapes".

also fixed silly "height" getter mistake to be the intended "color".
move creation of Doodles objects from inject.js to background.js

this makes the inject.js file just a simple library definition file
that can be (more easily) used outside of the extension context.
rename the library to "doodles.js" and update references.

this code is finally to the point where i'm pretty happy with it.
all the functionality i wanted is there. the code is well organized
and understandable and even a little bit documented. there aren't
any jagged edges or defects left screaming for attention. oh: one
glaring omission => automated tests. i should probably learn what
tools are available for javascript.