Add tests for JSON-LD parsing

- Tests whether the source of a document can be parsed to JSON-LD
- Test fails because the context does not work (see issue #4)
Edit TestJsonLd to run with different contexts

- runs with context "http://schema.org" for testing
- possible to run with local context file in project folder
- see issue #4
Use context of web application in TestJsonLd
Merge pull request #9 from SBRitter/context

Tests for JSON-LD Context