testng: Use custom reporter. Print summary and snippets.

Use custom reporter to detect feature failures instead of creating a new
runtime for each feature. Print summary and snippets at the end of the
run. Use feature files in src/test/resource/cucumber/runtime/testng
for the test of TestNGCucumberRunner.getFeatures().
Add new test runner class to examples/java-calculator-testng to show
how to run each feature as separate TestNG test by composition.