moved all the treemap javascript to separate file

seperate the js and html more clearly. This is in preperation for
serious work on #7.
moved all image fetching code out of treemap gen

this allows image loading to be done after the page structure is loaded,
meaning that on slower connections the treemap generation won't block
while it waits for the images. instead, the treemap is shown without
images, and then they are filled in as they are loaded. on a reasonable
internet connection this change makes no noticeable difference. Other
post-structure type page modifications should be added in the same way,
for the same reasons. that is, as seperate functions that are called
after the treemap is already created. This closes #7.