Write report stats to student's home directory

This script is run via sudo, so we need to get the name of the logged in
user to derive the path for the output file. Otherwise, it just logs to
root's homedir and that's not terribly useful.

Fixes #COURSES-554
Merge pull request #351 from binford2k/ticket/COURSES-554/write_report_stats_to_homedir

Write report stats to student's home directory