add set difference exercise
updated exercise to make more sense, and hopefully be more interesting
coverted tutorial to ipynb
reconstituted markdown version of shell tutorial
updated variables.ipynb
Converted tutorial to ipynb
updated data structures notebook
updated flow control notebook
Make the opening a little less scary
Add a Nano installer for Windows

1. Downloads and installs Nano into the users home directory
2. Adds Nano to the path
3. Makes Nano the default editor
replace richie with hopper
Switch from using .bashrc to .bash_profile nano path and editor export
Append exports to end of .bash_profile instead of overwriting

In the process at an explanation of where the lines came from
so that users know they can remove the lines if desired.
Remove testing line
Adding outline for DLA exercise, and renaming other files to make room for more
Updating index page
Correct variable name.
Merge pull request #42 from douglatornell/fix-stack-image

Correct variable name in stack frame image.
Merge pull request #7 from tshauck/set-difference

Add set difference exercise
Merge scopatz 'flow-cont' PR 21