Add stellar payments

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

Ack!  You shouldn't be overwriting a global variable in the constructor of an object.  This lets you affect the log used by all instances by creating a new instance and passing in some value for `config.logger`.

Instead, each instance should maintain a reference to the logger they should use and default to the standard one:

`this.log = config.logger || require('./logger');`