Release 0.5.6

[statrita2004]

In this release we include the following new features and improvements of ABCpy:

* We now allow the nodes of the Bayesian network to be multivariate random variables, previously only uni-variate random variables were allowed.

* We have provided some additional features for the output analysis, to access the posterior mean at different steps, access to distances computed etc.

* We have fixed some bugs related to transition kernels.

* We have corrected the discrete random walk kernel.

* Some improvements of the documentation.