Changed Rectangle to Shape

This will allow generalization of shape objects to include ellipses and
any other shape that might be needed.
Ellipses now render

Also,
Added layerPosition to clear up multi-shape layering
Added shape type: either "rectangle" or "ellipse"
Physics not working quite yet... not included in this commit
Touched up .gitignore

Added Thumbs.db
Added categories for clarity
Ellipses can now be fences

The ellipses.mml has been updated to showcase what is now possible using
fences.
Ellipse reflection was a complex problem that I solved with the ellipse
foci formula (conics) + the angle bisector theorem (geometry) + dot
products + transformations + distance formula + midpoint formula
Fixed white space
Added ellipse reflection interactive

Shows how particles reflect off of different ellipses at different
angles