NF: some metrics to report (vmem, pid) and control of logging from the environment

needed to troubleshoot extensive memory use while running benchmarks... so now it should be possible

VBENCH_LOGMETRICS=vmem,pid VBENCH_LOGLEVEL=DEBUG PYTHONPATH=$PWD nosetests -s -v vbench/tests/vbenchtest
ENH: vb_run_benchmarks -- use vbench's logging (if avail) and report time it took for a benchmark at debug level