Adding git graphical diff/merge tool lesson

Added a lesson for integrating _DiffMerge_ into Git's `difftool` and `diffmerge` commands. 
We've attempted to follow the sample repos/examples created in the previous lessons.
Also note that going through the mac instructions, you can't just copy and paste all the commands because some of them are on two lines and aren't properly escaped. I know this is not really something we can change, but it might be good to make a note of that because I'm positive that some people following along with this lesson will get tripped up on that.