Etl-toolbox

- Fix multiple bugs in ``merge_columns_by_label()`` that occurred with certain inputs
- Change ``merge_columns_by_label()`` to remove ``None`` and ``np.nan`` values from merged columns
- Change ``find_column_labels()`` to check whether the existing column labels fit the match criteria before searching rows
- Change ``map_labels()`` to return ``'-'`` instead of ``None`` for unmapped labels
- Change ``clean_whitespace()`` to return non-string inputs unaltered instead of raising an exception

- Added GitHub continuous integration
- Added project links and badges to readme and PyPI metadata
- Fixed bug in `merge_columns_by_label()` that raises a `ValueError` if `df` has multiple columns labeled `None`

Initial release