MemoryError on 32-bit systems

[readbeyond]

Due to the 2GB contiguous block allocation limitation of 32 bit systems, this line:

https://github.com/readbeyond/aeneas/blob/master/aeneas/audiofile.py#L447

might throw a MemoryError, causing an "unexpected exception".

We should:

1. catch it and log a meaningful error
2. document this limitation somewhere

[pettarin]

See #180 as well.