consistency across converters

Tried to make the different conversion scripts more consistent. Biggest change is that they all use the MiRNANode class now to add the miRNA to the network. This may result in different output than the original code: 1) all nodes now have the mature mirbas ID as identifier (before it was sometimes the hsa-mir-* form, other times the mirbase ID), 2) nodes that could not be mapped to mirbase are skipped with a warning in the log. Please see if this is OK for CyTargetLinker - i.e. if it doesn't break anything :).