Merge pull request #58 from nickdotreid/pip

rearranged index.html page
Save and next small change
week 11 initial release
Merge branch 'pip' of github.com:presnick/runestone into pip
session23 material on git
extract session codes from .rst files
bug fix
remove trailing colons on activecodes
week12 initial release
update week12.rst
session 25 material
sorting instances
minor fix to week12
Merge pull request #59 from aerenchyma/save-and-next

Save and next small change
removed chapters no longer used
week 13 initial release
Merge branch 'pip' of github.com:presnick/runestone into pip
week 13 reading responses