Add tabs list method (issue #152)

This PR is associated with issue #152. It adds the method #tabs_list to help make core module methods more consistent.

Output looks like this.
![breach-tabs-list](https://cloud.githubusercontent.com/assets/945367/3634869/7facb3d2-0f4e-11e4-8b49-b1a97df959e5.png)

#tabs_get method now calls #tabs_list when no arguments are provided.