ADD body placeholder with body= parameter on request

- only if it is an empty page (rev=0)

wonderful things can happen :)
Merge pull request #15 from jangxyz/ADD-placeholder-with-query

ADD body placeholder with body= parameter on request
Make mathjax block expressions left-aligned
Tweak CSS
Add comments to clarify nested loops
refactor models module into package

- still have some local problems in PageOperationMixin, but
- at least unit tests pass
one more step to better code

- but still far away