Correct issues with resizing of width on Rickshaw Preview

In util.js, don't hardcode a height (bug)

In vender rickshaw.js, correct some functionality to do with defaulting
sizes and establishing the preview sizes from changed configs.

In effect: either default height or set height makes no rendering errors
on width resizing.

Does not work in all cases however: logic doesn't allow for having no
default height/width and then setting one, since the magic
"(height|width)FromGraph" variable is only set once on create. Outside
of the scope of this fix.