Various changes. Sorry for the lack of multiple commits, still in the
early stages.

* We now use the g.gophe.rs/v2/icub3d/semver package for versioning.

* There are now some Cache DatastoreFuncs that are used, particularly
  in the larn section.

* Added a fetch.go file. It contains a wrapper around
  appengine/urlfetch that tries memcache first. Most of the urlfetch
  pages now use that (twitter being the exception).

* Added the packaging functionality for multiple versions.