export abn to "JSON abn" for BNviz compatibility

[matteodelucchi]

Export abnFit objects into an "abn-specific" JSON file that contains the complete information content to be used in BNviz.

Specification

graph:
   <string:value paris with information related to the whole graph>
nodes:
   <string, node_identifier>
      label: <string, node name>
      distribution: <string, node's distribution>
      parameterisation: 
         <array or string:value pairs with this node's parameters, which depend on its parents. Contains \mu, \beta, \sigma, \sigma_{alpha}>
arcs:
   source: <node_identifier>
   target: <node_identifier>
   frequency: <number, NULL. If available, frequency of arc appearance>
   significance: <boolean, NULL. If available, significant arc or not>
   constraint: <string of "retained", "banned", "free".>

Why not a standard BN format?

Many BN file format specifications are not suitable for storing information from continuous nodes. E.g., .bif, .dot.
An option is .net from HUGIN, which can store information from continuous and discrete variables (as mentioned in #63 ). However, it doesn't have specifications to store all "abn-specific" information.
We will provide a wrapper from the "JSONabn" to .net (and others) in the future.