print/logging messages in data loading functions

[dustinvtran]

All functions are silent whether it be loading, preprocessing, or saving the data. Currently, the only step that prints to stdout is downloading. These other steps can somtimes be very expensive, such as the preprocessing in small32_imagenet.py.

We should consider adding stdout messages to the loading, preprocessing, and saving steps—depending on how long it may take. And we should establish a standard that applies across all data sets.