Better examples

[fjarri]

Since the API of this crate is complicated, the examples will be the first thing potential users will look at. Currently they are quite messy and poorly documented.

What we need is:

• Extend the documentation, starting from README explaining which example demonstrates which functionality;
• Make the example protocols more realistic. Perhaps we can simplify the CGGMP presigning/signing, by cutting out all the Paillier stuff (this will be a good example for chaining protocols, too)? Or perhaps FROST (or a simplified form of it) will work?