parses string and calls methods for every tag and every not recognized
entity The characters < and > have to be written as < and > while
the & is written as &
The following three methods are called: defaultEntity(entity) for &
< > " ' entity(entity) for all other entities
openTag(tag) for all endTag(tag) for all text(text) for
all text
The startTag, endTag and text methods returns a string which is added to
the fully parsed string.