Support for SEP 10 challenge transaction

[ire-and-curses]

SEP 10 describes a specific form for the "challenge" transaction used for authentication. Providing a helper at the SDK level makes sense to save users from reimplementing the spec.

Here are example implementations in Go and JS.

[ramontayag]

Thanks for the heads up. Will be happy to review accept pull requests for anyone that needs this.

[jimdanz]

@ramontayag this can be marked as closed by #65

[ramontayag]

Yes thanks @jimdanz