ffmpeg.py does not check for unicode strings

[flokadillo]

In madmom.audio.ffmpeg.py, there are several lines where isinstance(infile, str): is used. We should replace this by isinstance(infile, (str, unicode)): to also support python 2.7's unicode string type.

[superbock]

Actually, it is only a single line (https://github.com/CPJKU/madmom/blob/master/madmom/audio/ffmpeg.py#L170), but using the above suggestion breaks Python 3, since it does not have unicode type any more.

Can you provide an example (or even better: a test) which shows the problem? I fixed something related quite recently #236. Seems like this fix was not enough.

EDIT: just searched for exact matches, missed many other instances.

[flokadillo]

Actually, this happens also in line 184, 243, and 292.
Here is a small example:
from madmom.audio.ffmpeg import decode_to_memory
infile = u'C:\\Users\\krf\\dev\\python\\madmom\\tests\\data\\audio\\sample.wav'
s=decode_to_memory(infile)
This produces the following error:
ValueError: only file names or Signal instances are supported as infile, not C:\Users\krf\dev\python\madmom\tests\data\audio\sample.wav.
Maybe we have to add an if clause to check types for different versions of Python...

[superbock]

@flokadillo, could you try the fix provided in #305 and see if it fixes your issue?