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.
Do not need try/except; use force_text() instead of smart_text().

Tested on older versions of Django, realized the try/except is
unnecassary. Should probably use force_text() instead of smart_text()
as the unidecode() functions requires an actual unicode string.
Only add unicode filename if the filename contains unicode.

To help some older browsers, such as Safari 5, the url quoted filename
is only added if the filename contains non-ascii characters. In the
ascii case, use the old style filename attachment. Fixed unittests.
Fix requirements so pip correctly installs Django and Unidecode.
Import external libraries inside def, avoids fixes setup.py.
Handle Django version 1.3.

Fallback from force_text to force_unicode as it is named in Django
1.3. Fixed the requirements in setup.py to reflect that Django 1.3 is
supported.
Added /build to ignore list
Version 0.3.3