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
replace richie with hopper
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
Merge scopatz 'vars-types' PR 18
Merge scopatz 'data-struct' PR 20
Merge scopatz 'strings-io' PR 19
added explanation about comparisons, genotype example details