Add .gitignore to root folder and remove matching files from repo.

[jdunns]

This fixes an issue where pulling the repo down fails on Windows:

\concept_checks> git checkout origin/master
error: invalid path 'unit_08/regression_fit_2d_exercise/Icon?'
error: invalid path 'unit_08/regression_fit_3d_exercise/Icon?'

The issue was caused by a carriage return character in the filename for these two files. I saw these files are zero-byte files and would match the .gitignore in another repo here. I went ahead and added the .gitignore and removed matching files.