Fix #115

Adds when_held event hook to Button (via extension of the EventsMixin
class). Also fixes some minor notes and activates codecov coverage
tracking.

.travis.yml

@@ -9,5 +9,11 @@ python:
     - "pypy3"
 install: "pip install -e .[test]"
 script: make test
+before_install:
+    # Coverage 4.0 no longer supports py3.2 and codecov depends on latest coverage
+    - if [[ $TRAVIS_PYTHON_VERSION == '3.2' ]]; then pip install "coverage<4.0dev"; fi
+    - pip install codecov
+after_success:
+    - codecov
 notifications:
     slack: raspberrypifoundation:YoIHtVdg8Hd6gcA09QEmCYXN