Story WYSIWYG editor and Attachments

Closes crowdcover/crowdcover#214
Closes crowdcover/crowdcover#295

Change the rich text editor on the `Story#edit` and `Story#new` pages to one based on the [Leptune's Editor](). The source for the editor is at http://github.com/bryanjswift/editor and is based on http://github.com/lepture/editor. I made some changes to the base editor and wanted to provide a point in time release of the code, so the version used here is also tagged at https://github.com/bryanjswift/editor/releases/tag/v0.1.1.

This series of patches includes the addition of a `StoryAttachment` model object and the corresponding database migrations. The `StoryAttachment` model is used when uploading images for `Story` instances. There is a placeholder delete method in `StoryAttachmentsController`. I wasn't sure if this functionality should be exposed or not, or under what conditions.