publish instructions for how to run tests

a node idiom (and hence a de facto package.json idiom) is that `npm install` should install all dependencies and then `npm test` would run the tests. currently when i try to run `npm install` it fails because dstore is not published to npm.

i'm really not concerned about what the exact method is to install the dependencies and run the tests but having a simple way of doing this helps with continuous integration and gives contributors a simple way to test their contributions.

if i can know how to run the tests i'll open a PR with a .travis.yml to get CI working.