TODO:

 o Ignore SYSTEM entities (first need to look if it's at all possible)

 o Allow tags which preserve spaces

 o consider every node consisted of only text and entities as final
   node (is it enough to use node.type == 'text'?)

 o fix normalizeString not to assume that inserting spaces is ok

 o add support for translating attributed (eg. imagine "title"
   attribute in IMG tag in XHTML)
