Update for latest jsTree version 3.1.1

[wakproductions]

Very nice gem! I made some modifications to bring your project up to date with the latest jsTree.

• Replaced static CSS files with the source .less files - this should make building new themes easier
• Updated documentation on how to incorporate themes
• Replaced legacy themes with the current themes distributed by the jsTree author
• Removed .DS_Store files from the git repository