misc additions to html serializer: javascript support, css etc.. 

@fosdev DISCUSS only. Initial commit:
- fixing misc issues found in previous commits (javascript);
- added rest client interface (question: to what type of requests headers and additional parameters must be added?);
- support for arrays of javascript urls & css urls to include in crichton.yml (we discussed that on friday) - please comment on that one in XhtmlSerializer.
- refactoring.