Initial version of the JavaScript client

[ffendt]

This PR will provide an initial version of the JavaScript client. It has still some open points before a release makes sense:

• Add JavaScript build to travis ci
• Prepare Jenkinsfile for release jobs
• see [JS] Add junit test reports #10 Add junit test report (either travis or Jenkins, or maybe both)
• see [JS] Add Server Sent Events (SSE) implementation for Eclipse Dittos events #8 Add SSE (Server Sent Events) implementation for Eclipse Dittos events
• see [JS] Unravel the WebSocket implementation #9 Add means to disconnect the WebSocket client (and vice versa should it should only connect after calling a connect() function)
• see [JS] Unravel the WebSocket implementation #9 Unravel the WebSocket implementation
• More tests (e.g. WebSocketClientBuilder, but first unravel the implementation)
• Compile with strict typechecking or users of the TypeScript definitions will fail to build unless they also disable strict typechecking.
• Migrate the tests of the api package from karma to jest
• add everything else as issues ...