Ffmpeg-normalize

* Properly detect -inf dB input.

* Debug command output for ffmpeg commands.

* Remove unneeded warning message.

* Use astats instead of volumedetect filter, fixes 163.

Allows floating point calculation.

* Print warning for bit depths > 16, addresses 163.

* Re-raise error on ffmpeg command failure.

This prevents incorrectly telling the user that a normalized file was written when it wasn't.

* Set tqdm lock for logging only when multiprocessing is available.

Multiprocessing is not available in all environments, for example
on AWS lambda python run time lacks /dev/shm, so trying to acquire
a multiprocessing Lock throws an OSError. The module could also be
missing in some cases (ex. Jython, although this library doesn't support
Jython anyway).

The solution to this is to only try to set the lock when multiprocessing
is available. The tqdm library solves this in the same manner.

For more details: https://github.com/slhck/ffmpeg-normalize/issues/156

* Add instructions on how to run tests.