Update README.md

Changing the link to grunt.js installation instructions.
Better error reporting for external Markdown files

1. Show an alert when loading the file throws an exception
2. Whenever the status isn't success (2xx) - upon exception or e.g.
  "file not found" - replace the section's content with information
  about the failure (instead of being silent about it and showing
  a blank slide).

We need to inform the user something went wrong, not just be silent
about it. An experienced developer, upon seing a blank slide, will
likely check the javascript console - but the rest will profit from
being told about the problem (and solution) explicitely.
More verbose error slide (HTTP necessary)
Add Reveal.js license to package.json
Merge branch 'patch-1' of https://github.com/jish/reveal.js into dev
Merge branch 'md-error-handling' of https://github.com/jakubholynet/reveal.js into dev
Merge branch 'master' of https://github.com/theoreticaLee/reveal.js into dev