RFC: Stateful Containers with tf.Module (#56)

* Create 20190117-tf-module.md

* Update 20190117-tf-module.md

* Loosen return type for variable properties.

* Use Dense consistently.

Thanks brilee@ for spotting!

* Remove convert_to_tensor from examples.

This wasn't ever required and including it might cause confusion.

h/t pluskid@ gehring@ and awav@

* Remove owned_* methods.

* Document `_flatten`

See tensorflow/tensorflow@5076adf6 for more context.

* Fix typo in module name.

Thanks k-w-w@!

* Update 20190117-tf-module.md

Author: tomhennigan