fix broken logic in TbHtml::addCssClass()

The pattern used in preg_match() doesn't work as intended since e.g. a
dash is treated as a word boundary. With this change the class string
is always operated on as an array and also removes the possibility of
accidentally ending up with duplicate class names.
fixed logic error which broke the build
Merge pull request #103 from Jalle19/refactor-addcssclass

Fix broken logic in TbHtml::addCssClass()