This commit allows badges to be generated with only a label or only
a value.
Badges must provide at least a label or a value, so it is not valid
to create a badge with no label and no value. This will result in
a `ValueError` being raised.
* 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`