Tensorframes

This is the final release for 0.2.9.

This is the final release for 0.2.8.

This is the first release candidate for 0.2.8.

Notable changes:
- uses the official java API for tensorflow
- support for image ingest (see inception example)
- support for Spark 2.1.x
- the same release should support both CPU and GPU clusters
- some usability and performance fixes, which should give a better experience for users

- uses the official java API for tensorflow
- support for image ingest (see inception example)
- support for multiple hardware platforms (CPU, GPU) and operating systems (linux, macos). Windows should also work but it has not been tested.
- support for Spark 2.1.x and Spark 2.2.x
- some usability and performance fixes, which should give a better experience for users
- more flexible input names for `mapRows`.