Added a plugin for drag nodes and modified 2 examples for see how to do it
Deleted an import from a HTML file
Fixed some imports in HTML files and an instatation of a `sigma` object
Added the declaration of a few JS objects
Fixed the size check and the dragging with rotated graphs
Now the  parameter isn't needed, it can be obtained from the  on renderer's .
Just corrected some typos
Updated NPM dependencies versions in package.json
Added current version number as sigma.version
Merge remote-tracking branch 'origin/pr/233'
Minor changes in sigma.plugins.dragNodes plugin

Disabled the plugin for WebGL renderers.
Removed the plugin from original examples, and added a specific example (drag-nodes.html).
Also updated README.md.