Removes db connection sharing between threads and adds database_cleaner

This runs much more stable, also when running in different environments like slow CI servers.
Convert CoffeeScript views to JavaScript to remove coffee-rails runtime dependency
Revert "Put an insert element button between and above each element."

This reverts commit 757be4229348c5d32972ec0b780457194305e7e7.

The new new Element insert Button has some serious design flaws, that have to be solved before we put this into master.
Fixes creation of external pages.

The presence validation of Page#urlname only should be triggered on update action.
Merge pull request #577 from magiclabs/fix/creating-external-pages

Fixes creation of external pages.
Use pre release gem in README Installation notes
Fix bug in jquery plugin for saving page sort order.

Apparently, the new page sorting algorithm was orinally made for Alchemy 2.6,
which used ```_recursiveItems(li)``` - in 3.0, the signature of that function had changed
to ```_recursiveItems(item)```. However, some of the code *inside* the function
did not appreciate that changed and barfed. I did not find this bug on my own, but had help
from  @acetoxy and @thereisnospoonpt (who wrote the original sorting algorithm patch). Thank you!
Bump version to 3.0.0.rc8
Refactors the dialog, so it is scrollable if the dialog content is larger than screen height.
Make all dialogs draggable.
Prevent scrolling of body, if dialog is open.
Updates Dutch translation with more translations, fixed some typo's
Convert item['slug'] into string before concatenating it with ancestor path.

Fixes #588