Add channel support to api.dartlang.org

This CL adds the following URL schemes:
  - http://api.dartlang.org/docs/channels/{be,dev,stable}/latest/
  - http://api.dartlang.org/docs/channels/{be,dev,stable}/424242/

Our channel builders are archiving the dartdocs to the following location:
gs://dartlang-api-docs/channels/{be,dev,stable}/REVISION/
Load all api documentation from the new channel locations

Changes include:
- Redirect old URLs to new channel documentation
- Update names for core libraries
- Update names for packages
Merge pull request #18 from mkustermann/channel-docs

Add channel support to api.dartlang.org