Use a unique coverage variable like istanbul command does

sandboxed-module looks for a specific global var that starts with '$$cov_'.  This patch uses that convention so sandboxed-module tests are also included in code coverage.  See https://github.com/felixge/node-sandboxed-module/blob/master/lib/sandboxed_module.js#L240 and https://github.com/gotwarlost/istanbul/blob/master/lib/command/common/run-with-cover.js#L147

Also a bit of tidying in package.json and jshintrc