add PySoundFile backend, and even make it default

[albertz]

On my system, it was using Gstreamer, which had several issues (e.g. #57, #62 and #63). Then I tried PySoundFile, which was even faster (0:04:31 vs 0:05:38), and does not have any of those issues. A related discussion for librosa is here. So, I think it would make sense for audioread to add PySoundFile as an additional backend, and even make it the default.

[sampsyo]

Sounds reasonable! Contributions along these lines would be welcome.

[shuguang101]

I have the same experience, PySoundFile is faster than Gstreamer. But i can't open libsndfile's web site, is libsndfile still being maintained?