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).
If there is no data, then the most likely situation is that the user have glyphs defined for individual points. Given the current design, the only think that can happen is to assume 1-length container and proceed. If we used generators instead of eagerly creating arrays, then we could defer decision regarding how many data points we have to generate. Note that switching to generators is not that obvious in JavaScript (we can't override index operator).