Update jq tree version to latest, restructure dirs

* Move vendor assets into app assets
* Update jq tree version to 0.18.0
* Update JQTREE_VERSION to 0.18.0
  - did _not_ update gem version(as stated in contrib guidelines)

Explanation:
I am working on a project(rails4) where I wanted to include jqtree; I found this gem and for whatever reason I could not get the asset pipeline to recognize the stylesheet and js imports in my manifests. SO, I took a look at the current state of things and decided to help out if I could by bumping the jqtree version and reorganizing a bit.

This may clash with how you would usually do things, so please let me know if I need to change anything to fit with how you would like to do things!