Customizable directory

1. Edit folder structure in configuration files:

  ``` yaml
  dir:
    model: /path/to/models
  ```

  or on initialization:

  ``` js
  require( 'coke' )( __dirname, {
    dir : { model : '/path/to/models' }
  });
  ```

2. Fix body_parser, session middleware.
3. Add `SCHEMA_DIR`, `MIDDLEWARE_DIR` to global variables.