#16 FIx download file names with special characters

[Anonymous]

The Content-Disposition header containing the filename must be utf-8 encoded as described in RFC5987.
The patch is an implementation of RFC5987 (found no library providing it correctly) and provides the encoding in this test: http://greenbytes.de/tech/tc2231/#attwithfn2231utf8 which works in all browsers (for IE at least IE9 is needed).
This patch makes it possible to download individual songs with correct filename. The problem of having incorrect filenames in zip files remains but that's a zip encoding issue.