Switch to beta only for "other" tiddlywiki releases

The current TiddlyWiki build process doesn't make alphas
at this time, so we don't want to confuse by using old
ones. This commit changes the code so it does the
right thing, but fails for the time being as the
generated externalized beta file points to the wrong files.

There are also no current test for an externalized stable
version, as that file does not yet exist (but hopefully will).

Tests associated with this process have been marked xfail,
and this is on a branch until resolution.

Part of issue #1060
Merge branch 'master' into betatiddlywiki

Conflicts:
	Makefile

Corrected to deal with change in curl downloads.
Allow an externalized non-beta tiddlywiki

The tiddlywiki build process now creates the necessary
files for both.
Add src/externals back in so that we can have meta files

We want to control caching on the associated files.
Ignore js files in src/externals, but not meta files