- Adds `zipline.pipeline.Pipeline`, a new user-facing class for managing
pipelines of Modeling API expressions.
- Adds `attach_pipeline` and `drain_pipeline` as API methods
- Removes `add_factor` and `add_filter` as API methods. These have been
replaced two new methods on `Pipeline`: `add`, and `apply_screen`.
- Adding a `Filter` as a column no longer implicitly truncates rows from
the Modelling API output. It simply causes a new column, of dtype
`bool` to show up in the output. Removal of rows is now handled by the
new `apply_screen` method of `Pipeline`.
- Refactors the existing Modeling API tests to reflect the new APIs.
