re-thinking model caching. this first commit removes a lot of old cached_ methods that essentially just cache their corresponding instance methods.

trello: https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching
model caching: replacing calls to cached methods in wingolfsplattform.
model caching: replace old cached_foo methods with cached(:foo) in your_platform.
model caching: fixing the usage of ActiveSupport::Concerns.

* http://api.rubyonrails.org/classes/ActiveSupport/Concern.html
* http://stackoverflow.com/questions/2328984/rails-extending-activerecordbase
model caching: fixing wrong use of .table_name.
model caching: first attempt to fix.
model caching: circumvent marshaling bug.

The problems we had, actually did not occur when converting an arel, but were due to a bug
described here:
https://github.com/mperham/dalli/issues/250

This circumvents this bug, but writing a cache takes a little longer.
model caching: clean up and correction to ActiveRecordCacheExtension cache deletion.
model caching: wingolfsplattform specs are green now, but pretty slow.
model caching: experiment with cache invalidation rather than cache deletion.

invalidation is much faster than deletion with wild cards (factor 8).
refactoring #cached method, which fixes most specs.
model caching: fixing some more specs.
new model caching: fixing the rest of the specs.
specs: introducing time_travel.

Usage: time_travel 2.seconds.
new model caching: implementing fill_cache method.
new model caching: merging in current master.
model caching: applying new model caching mechanism to list exports.

Trello:
https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching
new model caching: adding specs.

trello:
https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching
new model caching: adding `uncached` method.

trello:
https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching
new model caching: adding the cached { } block syntax.

trello: https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching

wiki: https://github.com/fiedl/wingolfsplattform/wiki/Caching