Numpy deprecation warnings

[maubuz]

When running sample script recording_examples/record_one_channel.py I'm getting the following deprecation warnings:

* recording
DeprecationWarning: The binary mode of fromstring is deprecated, as it behaves surprisingly on unicode inputs. Use frombuffer instead 
a = np.fromstring(data,dtype=np.int16)[0::2]

DeprecationWarning: tostring() is deprecated. Use tobytes() instead.
frames.append(a.tostring())
* done recording

The code still runs as intended but I think it would be nice to update it.

I've made the suggested changes and the code seems to run normally. I'll submit a tiny PR with the changes.