bug fixes (#12), Gemfile grouping, subdir feature (#13) and css cleanup

 * Update cancan, fixing Comment.accessible_by (fixes #12)
 * Group database adaptors mysql/postgresql/sqlite
 * fix path handling for subdir mounting (fixes #13)
 * css fixes: typo, duplicates, pre/code should be visible in preview too, remove font for comments
   * the comments should use the same font as the page they are embedded in