Add "Ball and Stick" Render option

[ghutchis]

The new default rendering style should be with balls and sticks:

http://3dmol.csb.pitt.edu/viewer.html?url=http://3dmol.csb.pitt.edu/tests/test_structs/benzene.sdf&type=sdf&style=stick:radius~0.15;sphere:radius~0.4

or..

http://3dmol.csb.pitt.edu/viewer.html?url=https://pqr.pitt.edu/static/data/mol2/WQ/WQLVFSAGQJTQCK-VKROHFNGSA-N.mol2&style=stick:radius~0.1;sphere:radius~0.35&surface=opacity:0.7;colorScheme:whiteCarbon

[RitwikGupta]

Saving this for later, because this makes everything ball and stick

pqr.threeDMole.all_viewers[0].setStyle({}, {sphere: {radius: 0.4}, stick: {radius: 0.15}});

[RitwikGupta]

See ad07351 for this WIP. @vonbearshark

[RitwikGupta]

[ghutchis]

I've changed my mind - I think the current look is different enough that people like it, so let's just add this as another option (i.e., not just sticks and spheres).