SEP-10 improvements

• SEP-10 improvement: adds the possibility to pass a client domain signing function to WebAuthenticator.jwtToken(...), so that the challenge can be signed by a client domain signer on a remote server without having to store the signing key within the app.

• ChangeTrustAsset improvement of sort order validation. See also: #167.

• Extend Network enum so that the network passphrase can be accessed.