Add examples to Pipes.Group

Hi,

I added some small repl-style examples for some functions, inspired by the `lens` package, because I find it useful to see a one-line usage example along the docs if possible.

I don't know what you think about this, but if appreciated I might do more examples for other functions and other `pipes-*` libraries in the next days in course of 24pullrequests.  

Let me know what you think about the idea, I am also open to any feedback!

[![24pullrequests](http://24pullrequests.com/assets/logo-625222452ffc0d57272decb6f851a7fe.png)](http://24pullrequests.com)

Best,
Markus