Create jshint task to be able to check js syntax

- Since we already have nodejs as dependency, we can
  make use of 'jshint', which is shipped per npm
- There are now two new rake tasks
- jquery.endless-scroll.js was moved to another folder, so
  it wont be handled by the javascript linter
Add js_syntax check to travis build
Merge pull request #65 from mafigit/add_jslinter

Create jshint task to be able to check js syntax.