Add stellar payments

- Adds core stellar payments library with client interface for adding payments to db.
- bundles mysql db layer

I do not expect a bare function to require a specific `this` binding...  I would rather you put this function on `Signer.prototype` and mark it as private by prefixing the name with an underscore like we talked about in the past.

If, at the very least you want to have it remain a base function, simply pass the signer in as a normal parameter instead of binding to it... way more straightforward.