don't upload if beforeEach returns false

check the value that beforeEach returns, if it is false don't
upload the file (similar to filedrop's functionality) and create
an 'UploadHalted' error.
Merge pull request #6 from davidlumley/check-beforeEach-response

check beforeEach response before uploading
- "headers" property allows interacting with Django backend, for example:

headers:
      'X-Requested-With': 'XMLHttpRequest' # Let Django show errors in AJAX-friendly manner
      'X-CSRFToken': window.getCookie('csrftoken') # Django CSRF protection

- use .shift() instead of .pop() on queues to let files upload in direct order
- allow dropping files while already uploading previous files
- imply "do not repeat youself" principle. Avoid duplicating code for drop and fallback uploads
Merge pull request #7 from AlexeyUzhva/master

Headers setting, direct file upload order
recompiled js