Improve global double buffer

I figured out how to include two buffer div's in a single markdown
preview view. I was still fireing markdown-preview:markdown-changed in
the previous commit, which triggers the MathJax engine to typeset the
page again. While removing this and including two buffers in the
preview (+ a third one that is visibility: hidden) it still doesn't have
the desired functionality. Lets lists the issues.

- If you have a long markdown source with lots of equations it is still
  possible to overload the preview st the preview to appear without any
  equations, and then they load, cause the loss of approximate scroll
  location as was previous reported.

  I don't know where the issue lies here. I have three buffers and the
  problem could lie between either MathJax typesetting the global buffer
  (render.coffee, 37) or it could lie with copying the content for the
  global back buffer, to the local back buffer in the preview
  (markdown-preview-view.coffee, 134). During the overloading the non
  LaTex content is visible, so it does suggest that the issue is with
  the global back buffer. That said I don't know how .html(<source>)
  works, if its a bredth of depth first build; the typeset html is very
  deep.

  On (markdown..., 134) I use .ready() to check for when the local back
  buffer should be ready. jQuery docs say that it executes after the DOM
  is ready, so I should work, but I would need to do more tests to
  verify this.

- If you have a preview window reopen from a previous program state, Dev
  Tools will report MathJax functions undefined. I have added call's to
  MathJax's functions without checking to see if MathJax's has actually
  loaded.

- Its incredibly dirty code, but this is purly an experimental branch
  with no thought to pushing for deployment, so thats ok ;D

I do know that for a transparent experience I need some sort of double
buffering, so we'll continue experimenting to find that, right now speed
is not the objective ;D