Add class representations

[hombit]

Implement __repr__ and __str__

[anlava]

Do we need both __repr__and __str__?

[hombit]

@anlava __repr__ implements __str__, probably __repr__ returning something like Amplitude(), Periodogram(features = [Kurtosis()], max_freq_factor=12.0) should be enough. However the last example shows that it may be really long, so it is worth to have a shorter representation given by metaclasses' __str__.