Makefile fixes

There's two changes here:
- Force npm packages to get installed in the current directory. I had
  jshint installed in ~/node_modules, so `npm install jshint` didn't
  do anything, and `./node_modules/jshint/bin/jshint` didn't exist.
- Force delete any files deleted on `make clean`, to ignore errors
  thrown when these files don't exist.