First messy implementation of detecting number of cores and parsing command-line options.
Refactored # cores detected code and startup message.
Read path to MNIST dataset from the command-line, instead of hard-coding it.
Pulled some re-usable command-line parsing and multicore-detection functionality out into app_utils module.
Just for fun, filter images based on training label and just show one hardcoded label. E.g. All sevens, for now.
Moved some WxWidgets-specific helper functions to separate wx_utills file.