Accept per-file headers in multipart file POST messages.

- Each file in the `files` argument can now be a 4-tuple, instead of
  just a file, 2-tuple, or 3-tuple. If it is, the last value in the
  tuple is a dictionary of extra headers.

- See [#1639](https://github.com/kennethreitz/requests/issues/1639) and [StackOverflow](http://stackoverflow.com/questions/19104924/how-to-add-per-file-custom-header-to-multipart-upload-with-requests/19105672#19105672) for more details.