.travis.yml, .coveragerc: use Coveralls.io to track tests code coverage

This requires one of the project owners (@astaric, @janezd,
@markotoplak, @mitar, ...) to flip a switch on
https://coveralls.io/github/biolab/orange3 or something.

.coveragerc

@@ -0,0 +1,5 @@
+[report]
+exclude_lines =
+    pragma: no cover
+    raise NotImplementedError
+    if __name__ == .__main__.:

.travis.yml

@@ -15,13 +15,15 @@ install:
       fi
 
 before_script:
+    - pip install coveralls
 
 script:
     - cd $TRAVIS_BUILD_DIR
-    - python setup.py test
+    - coverage run --source=Orange setup.py test
     - cd $TRAVIS_BUILD_DIR/doc && make html
 
 after_success:
+    - coveralls
     - if [ $TRAVIS_REPO_SLUG = biolab/orange3 ] && [ $TRAVIS_PULL_REQUEST = false ]; then
         source $TRAVIS_BUILD_DIR/travis/upload_doc.sh;
       fi