Add stellar payments

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

This block seems a little confused about the bindings..  `self === this` but you use the two alternatively in this chain.  IMO you should settle on one.

Additionally, you're using `Promise.bind`, but in all of the `then` blocks you're either re-binding the handler or using the closed over `self`, which doesn't make sense to me.  Either use the closure or bind and use `this`, but not both.