More Accurately Calculate Which Tags Are Popular

- When requesting tags, only those of a published posts will be
  returned.
- tag_model get_all_tags() will calculate the number of items tagged
  dynatmically.
- The column count of the tags table is unnecessary, the column and
  related code will be removed when the database migration tool is
  built.

Refs #38
Potential fix for page numbers > items tagged

- If tags were deleted but somehow the relationship still existed the
  left join would have counted those ones as well.

Refs #41
Merge pull request #41 from MitchellMcKenna/feature/more-accurate-tag-popularity

More Accurately Calculate Which Tags Are Popular