Issue #1 -- Efficient `stex` inner-mode detection

For math pre-rendering of LaTex formulae we need to figure out where
the `stex` inner-mode start and where they end. Thanks to CodeMirror's
`TextMarker`s it's possible to efficiently mark relevant parts of the
text with corresponding `stex` CSS and retrieve them later.
Issue #1 -- Look if *only* `stex` marker is at current location (instead of any) and if not (but still `stex` inner-mode) then figure out inner-mode boundaries.
sm update
Issue #1 -- LaTex formula preview, thanks to `MathJax`! ;-)

There are some glitches though: With the `.. math::` directive things don't works as nicely, since (a) it can be multi-line and (b) the `stex` inner mode is not recognized due to an unknown reason; fix! Plus the preview window should be *off* center.
Issue #1 -- Load MathJax on `beforerender`
minor
Issue #1 -- Refactored CSS & introduced `overflow: auto;` for `mathjax-box`
Issue #1 -- Ensured that `mathjax-box` is off center at the bottom right corner of the viewport.
Issue #1 -- Don't cover scroll bar with `mathjax-box`
Updated `sitemap.xml` to reflect new content
Merge branch 'master' of github.com:hsk81/notex