Add "make test" target to install prereqs for "npm test" to work.

Adds a "make test" target so that people don't need to manually figure out how to get "npm test" working.

Test plan:

 * cd to your rickshaw checkout
 * `make build`
 * `rm -rf node_modules`
 * Verify that `npm test` fails for lack of dependencies. If it doesn't you may need to remove the d3, jsdom or nodeunit modules from your global npm repository.
 * Run `make test`, verify that it installed the test dependencies and than ran the rickshaw unit tests.
 * Run `make test` again, verify that it didn't need to install anything and that it ran the rickshaw unit tests again.
 * Run `npm test` manually, check it still works ok.

Test plan works fine on my local machine. :)