Revise build script.
Revise build script.

- Check file's mtime to determine whether or not to build the theme css.
  This is quite important to improve the overall build process.
- Revise beforeExitSetup(), still has some problems
- Do not build semantic-ui's default theme anymore

Ref:
- Interesting links: https://gist.github.com/kethinov/6658166#gistcomment-1295057

semantic-ui-forest/forest-themes,semantic-ui-forest/forest-themes