Add 'AddSignature' function

[mootz12]

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

Change

There is a new function to help Sign a transaction. This function, Sign(string publicKey, string signature) is modeled after the following JS version: https://github.com/stellar/js-stellar-base/blob/master/src/transaction_base.js#L129

This function is extremely useful for Stellar Turrets, since they return a Base64 encoded signature and public key, and expect users to be able to apply those signatures to a given XDR.

[Kirbyrawr]

Looks good to me!

[elucidsoft]

LGTM, thank you!