closes#1818
Previously, the markdown of some chapters was converted to Jade markup
to support the conditional exclusion of TS prose parts in Dart
chapters. This commit reverts some of that back to clean markdown,
thanks to a few custom directives.
- Renamed `angular2_getting_started` to `angular2_quickstart` (to match
TS).
- s/-/_/g in project names elsewhere (only toh-5 & -6 needed to be
updated).
Fixes#378.
