Proto to embed assets and link to locally
Allow to embed the assets locally

It's then possible to pack the directory with the assets and the
presentation.html and to use it without having slippery installed.
Fix some style issue

* remove trailing blank (assets.rb)
* remove extra blank line at body end (add_impress_js.rb, document.rb)
* favor if for single line body condition (presentation.rb)
* missing space between { and | in block (add_reveal_js.rb,
  presentation.rb, processor_helpers.rb)
* space missing after comma (add_reveal_js.rb, presentation.rb,
  processor_helpers.rb)
* space missing before } of block (add_reveal_js.rb,
  processors_helpers.rb, assets.rb)
* space missing to the left of { block (add_reveal_js.rb, assets.rb)
* prefer ' (add_reveal_js.rb, add_impress_js.rb)
* use new Hash syntax (presentation.rb)

Thanks to PullReview.com/Ruby Style Guide
Add documentation to Assets module
Merge pull request #5 from toch/feature/local-asset

Feature/local asset