use textual css classes for issue priorities
see https://github.com/redmine/redmine/commit/113f8b5cf278ff32bd55bf63aafe11c3f5a085f1
1.1.2 from http://redminecrm.com/pages/a1-theme
1.1.3 from http://redminecrm.com/pages/a1-theme
merge original A1 v1.1.3 (from http://redminecrm.com/pages/a1-theme)
Merge pull request #1 from p-schneider/master

Master