Fix for miscategorisation of at-rules as rules (fixes #12)
Added new metric type 'atrule'
Refactoring of method runMetrics
Added support for media queries (fixes #12)
Added new Total Media Queries metric
Fixed a bug metrics measuring optional elements would break when those elements are not found in the stylesheet
Merge branch 'feature/fix-for-miscategorisation-of-at-rules-as-rules' into develop