Create an example notebook that shows training a classifier over the EWT dataset

[frreiss]

Create a notebook that is similar to tutorials/corpus/CoNLL_3.ipynb, with the following differences:

• The new notebook uses the semantic role labeling version of the EWT data set as its input
• The new notebook trains a token classifier over the upostag column

The new notebook should share the code in tutorials/corpus/util.py. Modify that shared code as needed.

[frreiss]

Closed by #198