Check if write() works by comparing the written file with an existing file

[mgeier]

... instead of reading it with read().

See discussion in #55.

filecmp.cmp() should probably be used for that.

And we have to add a 'peak' chunk to the file stereo.wav first.

[bastibe]

A more general solution would be a RIFF-aware comparison function, which only compares the contents of the fmt chunk and the data chunk. This should be pretty simple to do, and has less of a dependence on the implementation details of libsndfile.