Pipeline bugfixes, docstrings, and tests

[nelson-liu]

This PR fixes a few bugs in data.Pipeline, adds docstrings, and adds tests.

Bugfixes:

• Pipeline.__call__ was accepting positional args, but wasn't forwarding them to Pipeline.call
• In Pipeline.call, we were running self(token) for each token in an input list, which has the effect of running the entire pipeline on each token of the list. We were correctly executing only the current pipeline's convert_token if the input was not a list. This becomes a bug when you run a chained pipeline and the same step is executed multiple times.

Enhancements:

• Pipeline.add_after and Pipeline.add_before now return a reference to the Pipeline (so people don't run into errors doing pipe = pipe.add_*(something))
• Add a check to ensure that the pipeline's input convert_token is actually callable
• Replace lambda identity with identity static method, so we can pickle Pipeline objects.