Working revision for image with 4 heights

this will be replaced by a single height
Separation of <img> and <picture>

These HTML elements now both are implemented, since neither of them was able to handle all the required functionality.
Distinguish database for django-1.6 and 1.7
Finalized picture tag
removed dead code
Adopted to new picture plugin
Using metaclass to allow configurability of the ExtraFieldsMixin
SimpleWrapperPlugin uses the newly introduced metaclass
Removed useless imports
Added possibility to add an id tag to every element created by Cascade
It now is possible to freely configure sharable fileds for each Plugin
Enabling to display the ID of a plugin instance in structure view
template for images
style fixes
started to describe picture/image plugins after separation
subproject links using file cms_plugins.py now
All Plugin models now are generated on the fly

this has the huge advantage, that depending on the configuration settings, the models can be built in different variations
get_identifier now shows the ID of the element if available
use these settings to allow sharables and extra-fields
added get_identifier for carousel