Refactoring gruntfile

Removing stale code and unhelpful comments. Formatting. Build path
variable. DRYing up custom tasks. Adding clean task, so builds are
consistent.
Merge pull request #107 from doingweb/refactoring-gruntfile

Refactor Gruntfile
Spine CSS as a table of contents

Building Spine CSS as a table of contents from partials, so the
different parts are not obscured by multiple build steps. Additional
reorganization can happen in future PRs, so it's easier to integrate
changes at deeper levels, and so we get the advantages of smaller files
earlier.