Add `itertoolz.diff` to yield elements that differ between sequences.

Also, add `areidentical` to "Tips and Tricks" section of documentation.  `areidentical` *uses* `diff`.

This was first discussed in #200
This example feels a complex.  It requires me to think beyond just diff. 

Maybe the key function could be `str.lower` and we could compare a list of strings with various case differences?