Refactor repr_entries to expose ggplot-like interface.

Adding "additive representation" interface, akin to ggplot's additive
plot properties interface. EmbedReprModifiers can be added to embed, and
are responsible for setting appropiate repr entries to be passed into js
plotting layer. Adding composite modifier object to support storing
collections of repr modifiers without a target embed object, used to
define default composite representations.

Adding logging at debug level.

Updating examples with new interface.
Add Color, Clear, Helix, and Ribbon glmol_repr objects.
Upstream GLmol pdb parser updates.
Update glmol_embed __add__ to support repr iterables.
Scroll glmol view into focus on display.