Only error about missing fonts if text is actually drawn

Original [issue 132](https://code.google.com/p/plotinum/issues/detail?id=132) created by eaburns on 2013-07-08T11:26:19.000Z:

Currently, plot.New returns an error if it cannot find the default font. There are two concerns here: 1) if the default font will remain unexported then this might as well be a panic, and 2) we should support drawing plots that do not contain any text if the fonts aren't actually installed.

My plan is: export the defaultFont variable, leave the error return on plot.New, and change plot and vg so that, if the fonts aren't installed, they only error if/when text is actually drawn.