Stick RSpec to version 2.x

RSpec 3.0 has just been released. Since they use a new syntax now, many
tests broke. For the time being, stick RSpec to the latest 2.x version, no? :)

Thanks, and keep up the good work :)
Remove Alchemy::Page's dependency to have the user extend ActiveModel
Rename Alchemy::Page model concerns to avoid namespace conflicts
Bump version to 3.0.0.rc6
Make DummyUser class inherit from ActiveRecord again.

This reverts changes from 7b1e66940ff744535469f0bbeb563adadfa68f13
Adds DummyUser factories.

The user factories where removed with extracting the user model into it's own gem. But is useful to have them for the dummy users.
Adds controller spec for locking page feature at page edit.
Do not raise not found error, if a page user could not be found. Fixes #560
Bump version to 3.0.0.rc7
Adds available_locales setter to Alchemy::I18n module.

With this feature developers can dictate which translations are provided for the user in the backend. (I.e. if Alchemy had 10 translations, but the developer wants to provide only 2 of them.)

It can be used with a simple initializer in the host app: `Alchemy::I18n.available_locales = [:en, :de]`
Refactors clipboard. Fixes #539

The clipboard was an abstract wrapper around Enumerable. Since Rails 4.1 only allows to store simple objects in the session, it was necessary to refactor the clipboard to be a simple Enumerable again.
French translation
Removes mavericks phantomjs fix. Closes #554
Implemented a faster page sorting for language trees
Allow picture render size to be flexible in width or height

Before, you had to state render sizes in the format '300x200'.

Now, you can use e.g. '300' for inferring the height from the image file preserving the aspect ratio (or 'x200' for inferring the width).

These two additional ways of specifying the image geometry are provided by the used image processing library Image Magick (and its wrapper dragonfly), see http://www.imagemagick.org/script/command-line-processing.php#geometry.
Allow "20x" and "x20" syntax for resize, refactor PicturesController

1. The original PR proposed to make size: '300' indicate "infer height". in this PR,
'x300' maps to "infer width" and "300x" maps to "infer height". I find this slightly
more understandable (as the "x" indicates that something's missing, and which one).

The original tests had two conflicting test cases, one where size: '10' was to be
invalid, and one where size '40' was to mean 'infer height'. This is the point where
I had to change the API for this feature.

The other thing was that in the pictures controller, there was a ```#``` missing in a function
that is really just used for cropping with no crop_from data.

Refactoring:

* simplify and rename geometry_string functions
* there was some impossible to execute code (function takes mandatory args and then tests
  for their presence)
Fixing the bug in which the urlname of an invisible page would get corrupted while sorting and updated the tests
Removes translation keys that include interpolations.
Removes unneeded translations
Aligns en and nl to de translations