Add custom message when site in read-only mode

#4354 

When using read-only mode the admin will need to fill in a message in the UI:
![selection_064](https://f.cloud.github.com/assets/4497748/1638395/c001cf90-580a-11e3-908e-c2db66bb1bf8.png)

After saving the config, the user will see the following screen in the ``user`` and ``user/login`` pages:
![selection_065](https://f.cloud.github.com/assets/4497748/1638399/dbdf08e0-580a-11e3-9560-7dc2b5a21213.png)