adding a buffer reader

@hobu, we'd discussed this on the mailing list a year or so ago (http://lists.osgeo.org/pipermail/pdal/2012-May/000258.html). The idea is to create simple Reader based on an existing PointBuffer, which can then use the existing suite of Writers. I've got a limited set of test cases, which all pass, but there may still be a few warts.