Add stellar payments

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

if you set the `type` of the unirest request to json, you don't need to stringify your request manually.  from the docs:

```javascript
unirest.post('http://httpbin.org/post')
.type('json')
.send({
  foo: 'bar',
  hello: 3
})
.end(function (response) {
  console.log(response.body);
})
```