Properly handle platform name containing UTF-8 characters

On Fedora 19, python's platform name is something like "Linux-3.10.4-300.fc19.x86_64-x86_64-with-fedora-19-Schrödinger’s_Cat"

This causes a UnicodeDecodeError when running youtube-dl in verbose mode.

This commit fixes it by decoding the platform name properly with the util function decodeOption()