H5netcdf

- Fix test suite failure with recent versions of netCDF4-Python.

- Raise ``AttributeError`` rather than ``KeyError`` when attributes are not
found using the legacy API. This fixes an issue that prevented writing to
h5netcdf with dask.

- Include tests in source distribution on pypi.

- Add ``invalid_netcdf`` argument. Warnings are now issued by default when
writing an invalid NetCDF file. See the "Invalid netCDF files" section of the
README for full details.

- Fix garbage collection issue.
- Add missing ``.flush()`` method for groups.
- Allow creating dimensions of size 0.

- Datasets are now loaded lazily. This should increase performance when opening
files with a large number of groups and/or variables.
- Support for writing arrays of variable length unicode strings with
``dtype=str`` via the legacy API.
- h5netcdf now writes the ``_NCProperties`` attribute for identifying netCDF4
files.