first version of tests for expected RDF

it addresses #17 

not fully ready to merge but i would like to get some feedback, and discuss travis-ci integration
you can test it by running from root of the repository

```shell
$ npm install
$ npm test
```

i could also setup separate repository and run those tests independently!
I think instead of modifying the examples, it would be better to inject the context as [`expandContext` option](http://www.w3.org/TR/json-ld-api/#the-jsonldoptions-type) in the `toRDF` call. If you do it this way and a context already exists in the example, you'll overwrite it.