Upgrade less to 1.3.3.

Should improve the error message in https://github.com/meteor/meteor/issues/203
(see https://github.com/cloudhead/less.js/issues/990) though it may not fix the
way that the error is handled.
Switch less back to fully synchronous, now that the NPM module supports it.

syncImport was added in 1.3.2 and we just upgraded from 1.3.1 to 1.3.3.

Otherwise error handling gets pretty fussy (eg, sometimes less throws from
within a callback).
Document how to prevent your less imports from being double-processed.
Merge branch 'dev-bundle-less-upgrade' into devel
First draft contributing file. Basically just copied from the wiki.

Once this is released to master, we should update the wiki to point to it.
tweaks
Choose an even more portable CSS property to test @imports.

Colors were showing up as rgb(a, b, c) in some browsers and as #aabbcc in
others.
Explicitly make {$regexp: "foo", $options: "garbage"} throw.

This check was explicitly in the old selector compiler but the rewrite dropped
it. The `new RegExp("foo", "garbage")` throws on most browsers but not on old
WebKit.
Added fix, workaround handlebars issue and added tests
Use regexp instead of isNaN for detecting integers.
Move tests from handlebars to templating, as they depend on templates. Also, extend to test a few more types.
Review from Geoff
Merge branch 'pr-651' into devel
Merge branch 'devel' into ddp-pre1