Update to v7.0.1 & re-organize structure

This is for https://github.com/chug2k/nouislider-rails/issues/2 and includes updating the library to v7.0.1.

v7 refactored a lot of code into modules and moved libLink and wNumb into their own repos. I've re-organized the gem to better represent the file structure of the javascript library and changed the default requires to include all of the optional files, but listed steps on how to import the base files if you don't want everything.

One new feature of v7 is the Pips add-on (http://refreshless.com/nouislider/pips/) which is a legend/tick marks underneath the slider.