enable to handle multibyte filenames

I got tons of "Write failed: Wide character in syswrite at \
/usr/lib/perl/5.12/IO/Handle.pm line 207." errors when I tried to handle
multi-byte filenames through 'render_file' method.

Filenames encoded in UTF-8 can suppress these errors.
Merge pull request #4 from bicycle1885/pull-req-multibyte

enable to handle multibyte filenames