Build screenshots

On every build on the master branch (Squeak-4.5) a screenshot of Scamper is captured and uploaded to an Azure website. The screenshot history is kept for documentary purposes on that website and the most recent screenshot is embedded in the README.

**Implementation in detail:**

 - Added a test in the Scamper-Tests package to capture and save the screenshot. The opened website is currently http://google.de (can be changed easily). The test has no meaningful assertion and should not fail.
 - Added a script in build-support to upload the screenshot only if the SCREENSHOT variable is set, we're on the master branch and having a regular commit (no pull request)
 - Configured .travis.yml to call the script after a successful build
 - Added the uploaded image (https://swt-scamper.azurewebsites.net/screenshot/scamper.png) to the README