Bug: Non-200 HTTP Responses are returning an error

When uploading a file with a POST request, it is acceptable to respond
with a 201 created status code.  Dropzone was treating a 201 as an error,
this commit treats all 2XX status codes as a success.

References:
 * http://en.wikipedia.org/wiki/List_of_HTTP_status_codes
Cleanup: Change if to unless when looking for an error

As per the coding standards requested by @enyo
Merge pull request #57 from Gazler/non_200_success

Bug: Non-200 HTTP Responses are returning an error