Fixed renaming UTF-8  problems

When we had a filename with nonascii characters and when we were renaming the file it went broken giving a traceback like the following.  We fix that problem with this patch on the string_to_dict helper function.

Traceback:

    File "/Users/igortamara/.virtualenvs/co_web/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
      112.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
    File "/Users/igortamara/.virtualenvs/co_web/lib/python2.7/site-packages/filebrowser/decorators.py" in decorator
      65.         return function(request, *args, **kwargs)
    File "/Users/igortamara/.virtualenvs/co_web/lib/python2.7/site-packages/filebrowser/decorators.py" in decorator
      45.         return function(request, *args, **kwargs)
    File "/Users/igortamara/.virtualenvs/co_web/lib/python2.7/site-packages/django/contrib/admin/views/decorators.py" in _checklogin
      17.             return view_func(request, *args, **kwargs)
    File "/Users/igortamara/.virtualenvs/co_web/lib/python2.7/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
      52.         response = view_func(request, *args, **kwargs)
    File "/Users/igortamara/.virtualenvs/co_web/lib/python2.7/site-packages/filebrowser/sites.py" in detail
      493.                         redirect_url = reverse("filebrowser:fb_detail", current_app=self.name) + query_helper(query, "filename="+new_name, "filename")
    File "/Users/igortamara/.virtualenvs/co_web/lib/python2.7/site-packages/filebrowser/templatetags/fb_tags.py" in query_helper
      39.     add = string_to_dict(add)
    File "/Users/igortamara/.virtualenvs/co_web/lib/python2.7/site-packages/filebrowser/templatetags/fb_tags.py" in string_to_dict
      77.         string = str(string)

    Exception Type: UnicodeEncodeError at /admin/filebrowser/detail/
    Exception Value: 'ascii' codec can't encode character u'\xf3' in position 26: ordinal not in range(128)