Adding npm commands

These are just some good hygiene points that make it easier for other developers to work with this project.

* Defining `npm test` makes project adhere to the npm standard that some meaningful tests will be run via `npm test`. This makes it easy on CI, and makes it easy for developers to enter the project and not have to dig around in a Gruntfile to figure out which tests they should be running. (I picked `grunt karma:travis` but let me know if something else would be better.)
* Adding a `postinstall` step so `npm install` will do both `npm install` and `bower install` - one less command to remember, and one less thing for people to screw up. (Particularly new people who may not immediately realize that bower is necessary.)
* Adding `'use strict'` at the top of the Gruntfile to adhere to that best practice.

Thanks!