Release of tensorflow-io v0.15.0

[Yong Tang]

Hi All,

We are happy to announce the release of tensorflow-io v0.15.0, available through PyPI.org (`pip install tensorflow-io`)

The v0.15.0 release is compatible with the new TF 2.3, and comes with new features such as initial support for GCS/TPU, null handling support for AvroRecordDataset, GIF encoding (encode_gif), as well as a complete list of APIs for colorspace (YCbCr/XYZ/LAB/etc) conversions.

tensorflow-io v0.15.0 support python 3.5, 3.6, 3.7, and 3.8 on Linux/macOS/Windows.

All python wheel packages are available to be installed with:

```

pip install tensorflow-io[==version]

```

Please see more details from the release notes at the end of the email.

Thanks to everyone who contributed to and supported these releases!

# Release 0.15.0

## Major Features
* Add basic gstpu file system for GCS/TPU support.
* Add null handling support for `AvroRecordDataset`.
* Add tfio.experimental.color for colorspace conversions:
  - `tfio.experimental.color.[rgb_to_bgr|bgr_to_rgb]`: BGR
  - `tfio.experimental.color.[rgb_to_rgba|rgba_to_rgb]`: RGBA
  - `tfio.experimental.color.[rgb_to_ycbcr|ycbcr_to_rgb]`: YCbCr
  - `tfio.experimental.color.[rgb_to_ypbpr|ypbpr_to_rgb]`: YPbPr
  - `tfio.experimental.color.[rgb_to_ydbdr|ydbdr_to_rgb]`: YDbDr
  - `tfio.experimental.color.[rgb_to_xyz|xyz_to_rgb]`: CIE XYZ
  - `tfio.experimental.color.[rgb_to_lab|lab_to_rgb]`: CIE LAB
  - `tfio.experimental.color.[rgb_to_yiq|yiq_to_rgb]`: YIQ
  - `tfio.experimental.color.[rgb_to_yuv|yuv_to_rgb]`: YUV
  - `tfio.experimental.color.rgb_to_grayscale`: Grayscale(BT 709)
* Add `tfio.image.encode_gif` for GIF (animated) encoding support.

## Thanks to our Contributors

This release contains contributions from many people:

Cheng Ren, Paul Shved, Vignesh Kothapalli, Yong Tang, marioecd

We are also grateful to all who filed issues or helped resolve them, asked and
answered questions, and were part of inspiring discussions.