Add package testing to CI (#68)

* Make `build_examples.py` callable from python
* Allow tests to run from outside project directory
* Add invoke tasks
* Add server tests
* Run travis tests against wheel package instead of local code
* Update `badge.write_badge()` to support `pathlib.Path`
* Update `CONTRIBUTING.md`

build_examples.py

@@ -1,14 +1,13 @@
 import anybadge
 
-if __name__ == "__main__":
 
+def main():
     print(
         """
 | Color Name    | Hex     | Example |
 | ------------- | ------- | ------- |"""
     )
     for color in sorted(anybadge.colors.Color):
-
         file = "examples/color_" + color.name.lower() + ".svg"
 
         url = "https://cdn.rawgit.com/jongracecox/anybadge/master/" + file
@@ -20,3 +19,7 @@ if __name__ == "__main__":
         print(
             f"| {color.name.lower():<13} | {color.value.upper():<7} | ![]({f'{url})':<84}|"
         )
+
+
+if __name__ == "__main__":
+    main()