Add stellar payments

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

IMO, we should have notes regarding that on a function that performs the normalization:

```javascript
//knex doesn't return bluebird promises, so we normalize db results so we are always using
//bluebird promises
var toBluebird = Promise.resolve;

...

return toBluebird(...)
```