Support for reload, style improvements

- reload the present., keep the current slide on SPC
  => useful for previewing it while developing it
- moved the background div to the background
 not to prevent copying and clicking on the content
- style: brownish color for code to make it stand out
- readme improvements
Updated Showdown to 0.3.1, info about Sh. extensions

- updated to 0.3.1
- added info about custom and existing Showdown extensions
- links to related tools (PageDown, reveal.js)
Merge pull request #1 from jakubholynet/master

Support for live reloading, Showdown upgrade, fixes etc.