Add stellar payments

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

I think I realized what is wrong with this. It constructs a ```Promise``` just to use its ```resolve``` method instead of using the static method ```Promise.resolve()```.

```js
database.prototype.getUnsignedTransactions = function (limit) {
  var dbPromise = self.db('Transactions')
    .where({txblob: null, submittedAt: null, abortedAt: null, error: null})
    .limit(limit)
    .select();
  return Promise.resolve(dbPromise);
};
```