Added an Icon (Mac-specific): do not track (.gitignore)

Credit to this post for explaining how to make it work:
http://blog.bitfluent.com/post/173740409/ignoring-icon-in-gitignore

Signed-off-by: Jonathan A. Whiteley <jonathan.whiteley@mail.mcgill.ca>
TODO; start investigating calculating Effect Sizes for nested ANOVA

Effect Size (ES) for factor = SS_trt / SS_total
- I just have to figure out how to add that to the summary() output as an additional column (?)

Signed-off-by: Jonathan A. Whiteley <jonathan.whiteley@mail.mcgill.ca>