Add help output to the Makefile

This is now output when running `make` on its own. It was the first thing I ran when pulling the project and I wanted to run the tests.

```
$ make
Usage instructions:
    make lint                 runs jshint on the source code
    make test                 runs the test suite using phantomjs
    make build                creates a production (minified) build
    make help                 displays this message
```