fix issue with overlays for #419; unfortunately have to fall back on rectangle shape for overlays due to issues w/ transformation matrix & node centres not lining up to centre of shape if adjusted to fit in bb
initial refactor of tests
cytoscape.js pulled into mocha tests
better error message on bad edge json
null renderer & layout used automatically if no html dom ele specified
all init tests ported to mocha; tests can be run in nodejs or the browser; tests integrated in make process; just need to port other test modules