Add examples for common usecases

[vkomenda]

An example is given in #8 for cryptographic signature verification. Further examples that can be added for the API functions may include:

• message encryption with PublicKey::encrypt and decryption with SecretKey::decrypt,
• for a threshold scheme, cyphertext share decryption with SecretKeyShare::decrypt_share and decryption of the entire encrypted message with PublicKeySet::decrypt,
• generation of threshold signature shares, their combination into a signature with PublicKeySet::combine_signatures and verification of that signature with PublicKey::verify.

In addition, poanetwork/hbbft#183 (comment) provides a shared randomness usecase for an internal computation of threshold_crypto that only uses DecryptionShares and removes the xor_vec step that would require a Ciphertext. With a slight modification of the API, such an example can be provided as well.

[DrPeterVanNostrand]

I started working on some threshold signing and encryption examples on Friday, I can take this issue.