SEP-10: muxed account & memo support

[JakeUrban]

Updates the SEP-10 utility functions to support muxed accounts and challenge transaction memos as specified in the SEP PR

Updates

• Updated utils.buildChallengeTx() to accept muxed accounts (M...) for client account IDs
• Updated utils.buildChallengeTx() to accept a memo parameter to attach to the challenge transaction
• Updated utils.readChallengeTx() to provide a memo property in the returned object
• Updated utils.readChallengeTx() to validate challenge transactions with muxed accounts (M...) as the client account ID

[leighmcculloch]

Thanks for adding this capability to the SDK. Two questions (❓).

[Shaptic]

I can't validate the SEP-10 stuff, but the JS itself LGTM! Minor notes follow.

[JakeUrban]

If there's urgency to merge this before the bump to js-base, can you file an issue so this doesn't get forgotten? I'd prefer to have it use the builder

@Shaptic this isn't urgent but I wouldn't want to wait long for the js-base change. Can we make that change and any other withMuxing additions needed throughout the base project within the next week or so?