Add allowed values for fields in OPTIONS

This is a naive implementation that looks at the field's widget, and if it's not an autocomplete, it exposes the allowed values using ``_options_get_options``. This will serves for both entity-reference module, and taxonomy term reference.

term reference:
![postman-3](https://cloud.githubusercontent.com/assets/125707/4850977/d87e6d36-606e-11e4-8bd2-2b5447de8c8e.jpg)

ER:
![postman-2](https://cloud.githubusercontent.com/assets/125707/4851038/418be3da-606f-11e4-817b-43d2c27d8f3d.jpg)

We will also want to test:
  - Taxonomy term fields. With hierarchical information.
  - Option fields (one type is probably enough).