Break out SimpleShortestPathVertex

SimpleShortestPathVertex still has its own

putformat and can't be directly called from the bin/giraph command line. This makes it only useful to data stored in json format and, as an example, can (and has) made people think they need to implement their own *putformats for each vertex. The vertex itself should be made generic, the json formats should be split into standalone formats, and the ability to run this directly from bin/giraph should be verified.