Store qe_stats_file in output directory (#262)

Function log_execution_env_state is used to gather information about the execution environment and store this together with the experiment log.  Recently we've added saving the compression schedule YAML file in the same logs directory.
This commit expands the log_execution_env_state interface to accept a list of paths to arbitrary files that may contribute to the experiment configuration and that you (the experiment owner) deem important for recreating the experiment.

In the sample classifier_compression.py app, we now store both the compression schedule YAML file and quantization statistics collateral file (qe_stats_file).