simple-cipher: fix the description

[jtigger]

The framing for this exercise can be frustrating, confusing. Here's a laundry list of things that, if fixed, would improve that experience:

• The reference to image of Caesar Cipher is broken. If that plotline is kept, fix the reference (e.g. link to a stable source).
• The goal and supporting plotline are a bit scattered. There's a hint of a plotline of keeping your little sister from reading something; perhaps go all-in on that narrative and trim off others?
• This exercise is suddenly overwhelming in size with no explanation. Motivate the 3 iteration exercise — or at least telegraph the general structure of the exercise.
• The description runs-on into the the concatenated "how-to-run-the-tests" chunk that follows. Given that this is a multi-parter, put an ---- at the bottom to separate the last section from track-specific instructions.

[ldwoolley]

I would like to add an issue here. Currently the simple-cipher assumes that crypto-square will follow. Not all tracks javascript#312 order the exercises this way. I know mine is a simple change, but if there are no arguments against the proposed improvements from @jtigger, then I will give all the points a go. Please let me know if there are more burning complaints with the current format of this exercise, so I/we can implement them now.

[hilary]

Here's another concern. The description presents the exercise as a Caesar Cipher, but in fact the tests (and much of the description) are for a Vigenère Cipher. In Step 2 and 3, the problem refers to the cipher described as a substitution cipher, which is technically correct but not particularly helpful. The cipher is a polyalphabetic substitution cipher (specifically, Vigenère).

[hilary]

Another issue is that there's an error in one of the examples.

In Step 2, the problem reads

Given the key "ddddddddddddddddd", encoding our string "iamapandabear" would return 
the obscured "lpdsdqgdehdu"

The return value given is incorrect. The correct value is "ldpdsdqgdehdu" (note the missing d at index 1).