Only link sites in dashboard widgets, if login_url is present.

Fixes #651
Replace CanCan with CanCanCan
Use full asset url for tinymce base path, if asset host is set.

Closes #596
Complete new README

Added much more useful information into the readme that should help to get started with Alchemy even better. Closes #632
Set positions on elements manually while ordering

We do not use acts_as_list methods anymore which saves database quaries. This change also ensures the correct position of the elements.
Fixes "publish page" specs