added popular method

Feature added to find popular tags

Refractored using new lambda syntax, added 'least' method

Removing white space

Fixing test so that taggable counts are set in rails 3.2

Restoring space for readability

Included information on new features 'most_used' & 'least_used'

Editted to display properly
Merge pull request #577 from wegotcoders/popular-feature

Popular feature