Assets tweaks

Alright, so i noticed there is no obvious way to minify JS files, so I added a Gruntfile to easy do exactly this.

Additionally:
- i removed a potential warning in case of `NINJA_FORMS_JS_DEBUG` set to true in `wp-config.php` (btw, any idea why `SCRIPT_DEBUG` is not used?);
- scripts can be also linted (but since there are quite few warnings, this is disabled)
- i did a little cleanup in js files: plugins are now put in `js/XXX/vendor` (where XXX is either min or dev);
- I added unminified version of js plugins used;
- file names doesn't include a `.min` suffix anymore (all files in `/js/min` folder are minified);
- [sourcemaps](http://www.html5rocks.com/en/tutorials/developertools/sourcemaps/) are here! 
- running `grunt` or `grunt dev` will also remove all files in `/js/min` files. Useful to clean up leftovers (there was a difference of 3-4 files between `/js/dev` and `/js/min` folders).

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In addition, for those who doesn't know,[Grunt](http://gruntjs.com/) do a lot of other stuff: minify css, optimize images, generate sprites and a lot more (basically any repetitive task you can think of)