Tasks: Expose check-modules task
Tasks: Add clean-dist task
Tasks: Add lint and build-wordpress tasks

Defining a standard build-wordpress task removes some boilerplate from
every site's Gruntfile.
Tasks: Change build-pages to build-posts and handle all post types

Changes preprocessPost() export to postPreprocessors hash.
Allows specifying the target file name via a preprocessor. This was needed by
meetings.jquery.org to support the custom directory structure.

Fixes gh-35