Work on #87

Implement a test suite, including Travis-CI integration

coverage.cfg

@@ -0,0 +1,20 @@
+[run]
+branch = True
+include = gpiozero/*
+;omit = */bar.py,*/baz.py
+
+[report]
+ignore_errors = True
+show_missing = True
+exclude_lines =
+    pragma: no cover
+    def __repr__
+    if self\.debug
+    raise AssertionError
+    raise NotImplementedError
+    if 0:
+    if __name__ == .__main__.:
+
+[html]
+directory = coverage
+