Commit for bootswatch3 version

This commit contains the *.scss files for bootswatch3.
All the source files (*less) are at /bootswatch3 (manually copied).
All the converted files (*.scss) are at
/vendor/assets/stylesheets/bootswatch3/<theme-name>/

Other changes:
1. Update Rakefile, rearranges to include bootswatch3, and version.rb,
CONVERSION.md and HISTORY.md to match these changes.
2. Included default theme and removed spruce and superhero from the
theme list.