Rename "sliderWindow" in Visualizing Derivatives to "slidingWindow"

Auditors: eater
Growth mindset A/B test -- attempt to include change to khan-exercise.js.

Test plan:
Ran on appserver on local machine.

Review:
Jace
fixed spacing
Revert "fixed spacing"

This reverts commit 96834827e24c32ef38d31099c9bf9140164256cb.
Revert "Growth mindset A/B test -- attempt to include change to khan-exercise.js."

This reverts commit b280848971bf4b1e23bf7697349ee78180d6e065.
Remove old exercises which aren't currently live.

Summary:
Looked for files which didn't match a name that @eater listed.

Removed any which haven't been touched after August 2012.

Test Plan: none

Reviewers: eater, alpert

Reviewed By: eater

CC: tom, stephanie, emily

Differential Revision: http://phabricator.khanacademy.org/D1056
Remove spurious change from investigating visualizing derivatives on IE.
made a fraction with both negative numerator and denominator reduce to both of them being positive (ex: -2/-3 = 2/3)