Add glyphs/sprint example

This (almost) reproduces http://www.nytimes.com/interactive/2012/08/05/sports/olympics/the-100-meter-dash-one-race-every-medalist-ever.html.

Besides the example this PR:
1. Adds support for number formatting to hover tooltips.
2. No more `Text(angle=0)` (added default value).
3. Adds `Text.{x,y}_offset` (`screen` units by default).
4. Adds `Plot.add_glyph(glyph)` syntax (`source` is optional).
5. Removes dummy data sources (this PR and a few older ones replace PR #1052).
This seems to have the effect of stipulating that the length is 1 whenever there is no data?