Reuse go.talks templates and static files.

Update server to use templates and static files directly from
code.google.com/p/go.talks/present.

Move talks to talks.godoc.org so we can match the directory structure of
the present tool without conflicting with godoc.org's own directory
structure.
Use godoc.org specific user agent string.

To satisfy Github requirements for user agent strings starting in March
2013, specify a godoc.org specific user agent string when requesting
files from version control services.

This change also fixes #60.
Presentation improvements

- Enable play.
- Use a cache to avoid multiple fetches of the same file when building a
  presentation.
Add dependency graphs.