Added support for non-ascii filenames in content-disposition.

Using the details outlined at:
http://kbyanc.blogspot.hk/2010/07/serving-file-downloads-with-non-ascii.html

To support older browsers, an ascii only filename is added to the
content-dispostion. The unicode version is properly encoded and added
to the header. The argument attachment_filename may now be False to
send no filename. Added and fixed unit tests to verify the new
behavior.

This change introduces a new dependency on Unidecode <https://pypi.python.org/pypi/Unidecode>. I have tested this change with Python 2.7.5 and Django 1.6. All unit tests and manual test cases worked for me. Please review the changes. If any other configurations do not work please let me know with version numbers.