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).
6. Makes `Plot` maintain order of renderers (#696).
please make this change... it is really better, and I would also say it would be great if you have axis labels... it helps the reader to quickly see what's the meaning of the axis (y is understandable because they are years, but x are not understandable at first sight).