Fold scale_info into scale objects
Default scales don't make a plot dynamic
Preserve class of merged scales
Allow adding scale objects with just a title
Allow mixing of scale types, as long as they're in the category
Fix naming of axes with layers
collapse_ggvis_scales: code cleanup
Add functions to create sparse scale objects

This makes it possible to apply scale defaults after collapsing the
scale objects, instead of before. Also, this commit adds a distinction
between scale names (like "x", "y", "fill", "foo", "bar") and
visual properties that the scales can apply to ("x", "y", "fill", but
not "foo" and "bar"). Note that these properties are different from
props, which also can include "x2", "y2", etc.
Use explicit arg for setting scale label
Add missing shiny::
Fix example
Update scales test
Use correct automatic scale labels
Simplify test
Update test specs
Re-document
'font' is a valid scale
Allow utc to be NULL
Update scale_singular for new interface
Re-document