Figure out a good way to render text in data units

[AndrewRook]

Right now bokeh's text glyphs only work in screen units, not data units. However, there are several things that ptplot does or will support that should have text in data units, like football yardline numbers or jersey numbers. Right now ptplot uses a combination of kludges — making a static image with matplotlib for yardlines and having an empirical heuristic for jersey numbers (although the latter doesn't handle the case when a user wants to zoom in...).

Couple of possible options for solving this:

• Get a reliable way to translate text into a set of coordinates that bokeh can render (maybe an SVG path, or a complex patch object). I did some work in this area as part of the initial (plotly-based) release, but ran into trouble when it came to automating the work.
• Figure out how to set up callbacks to change font size when a plot is zoomed, and figure out how to ensure that plot dimensions always stay fixed even when zoomed (so you can't make a skinny vertical slice of the field and throw off all the relationships between the horizontal and vertical scales.

[AndrewRook]

For the callback approach, see here for some implementation ideas.