[#9] added cancan permissions for guest users (members of the public)
[#9] Access to unpublished articles handled by cancan rules.
[#9] Set the flash[:info] when rendering unpublished articles
[#9] Render missing article template when access denied.
[#9] flash[:info] messages rendered in the article sidebar.

Includes styling from Twitter bootstrap for alerts. I checked a box
in the custom Bootstrap form for "alerts" and replaced the resulting
bootstrap.css.

Limitations:

Does not include 'close' button to close the flash message.
[#9] Added links in admin panel to preview unpublished articles

'Open' opens the article on the site if it's published
'Preview' opens the article on the site if it's unpublished
'View' shows you the raw data of the article in ActiveAdmin
'Edit' opens the article for editing in ActiveAdmin
'Delete' destroys the article

Whether these links show or not is handled by CanCan in the Ability
class
[#9] flash[:info] messages on articles can be dismissed.

Rather than include all the button and form styles from Bootstrap, this
just copies over the relevant rules for alert buttons.
[#9] Remove render_markdown check in GuidesController

Guides have always been written in markdown; there is no show_html
template to render.
Merge branch '9-cms-preview-draft-articles'