update rspec syntax in shared examples
Switch shared_examples specs to `let` and update contextx

`let(:user)` is now used instead of @user in shared examples
contexts were updated not to repeat user twice everywhere
Move ActiveRecord specs to new Rspec syntax

* syntax was changed
* instance variables were replaced by `let`
* formatting was updated
extract controller specs to shared examples
Unify and simplify behaviour for controller activity logging specs

One thing I don't understand yet is why MongoMapper does not run this suite and
why, when I tried to run it, it failed
improve test names for AR and DM
remove rails3 spec option (not used anymore)
update test descriptions for mongoid
update test descriptions for mongo_mapper