Anchor fix, and other extrinsic clean up

* Strips punctuation from all text before fingerprinting (for `full`, `kth_in_sentence`, and `anchor`)
* Uses utility functions provided in the `ExtriniscUtility` class defined in `shared/util.py` to access `sample_corpus` listings and our training sets. This also fixed some relative import issues (since we were previously running `python extrinsic_testing.py` and are now running, from a different directory, `python -m plagcomps.extrinsic.extrinsic_testing`