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