README.md: only one timeselector per page

++ What: Adding section "Restrictions" to make prominent that you can only have one timeselector per page
++ Why: Beside reporting the stated fact as an issue it should be kept in mind for implementing timeselector for the time being
jquery.timeselector.css: binding styles to div

++ What: ```timeselector {``` => ```div.timeselector {```
++ Why: These styles are meant exclusively for the div that houses the + / - for time selection, making the deep connection to that div explicit allows using independent / more general ```timeselector``` class(es) (defined somewhere else), esp. giving the corresponding ```input``` element the class "timeselector" without being forced to state explicitly ```input.timeselector``` (which would make no sense if you want to say very general "this input belongs somehow to a timeselector context, details open" - but not (as for div.timeselector) "this input should behave in a very, very special way")
++ Note: changing to ```div.timeselector``` is "cheaper" than changing to something like "timeselector-div" which would require to change also the JS code
Merge commit '42b93cbd449da38200edccf85c4b989ceff4e8f9' into no$

Conflicts:
	README.md