Drop buggy appcache path route support

Despite best efforts, we've failed to get URL path routes to work bug free across browsers.

Path routes use the path part of the URL to express "where" or "which page" is being displayed in the application, such as http://example.com/one and http://example.com/two.

The attempt was to support path routes with a application cache manifest fallback entry of "/ /", which caused paths which didn't otherwise match to fall back to the root URL "/", and so load the application HTML.

However Chrome at least adds each and every such encountered path to the manifest as a "master" entry. This slows code update reload times (Chrome will re-download each master entry), and has the danger of
eventually running out of space in the application cache (which will cause code updates to fail).

See https://groups.google.com/forum/#!topic/meteor-core/bUTOXugc5qE and
https://groups.google.com/forum/#!topic/meteor-core/tFXZkhIWeKk
for discussion.

The current best known approach is to use hash fragment routes instead when using the application cache.

For example, http://example.com/#!one and http://example.com/#!two.

The Iron Router package plans to automatically support hash fragment routes when the appcache package is used.  (EventedMind/iron-location@ea5085d)

I've also renamed the appcache-* (with a dash) source code files to appcache_* (with an underline) for naming consistency with the other Meteor core packages.