Odds and ends commit

A bundle of very small, and mostly non-functional, changes to the code.
Mostly they are unrelated to each other
../../../distiller/apputils/checkpoint.py – add info to exception

../../../distiller/apputils/image_classifier.py – remove unused
`--extras` command-line argument

../../../distiller/thinning.py – code refactoring (non-functional)
except for adding a new public API: contract_model()

../../classifier_compression/compress_classifier.py – use
contract_model() when using `--thinnify`

../../lottery_ticket/README.md – remove illegal characters in
the text