Add pybind11 wrapper for decoder

[kuke]

Resolve #735

[kuke]

Example usage:

in the decoder directory

$sh setup.sh
$python
>>> import decoder
>>> import numpy as np
>>> probs_mat = np.array([[0.1, 0.9], [0.5, 0.5]])
>>> decoder.decode(probs_mat)
u'example decoding result'