Structure tags and no restriction on allowed prop. tag attrs.

This PR allows structure tags to be defined in the structure template
file. These tags can subsequently be used by other bundles.

It also removes the unique priority constraint on tags and adds an
"addTag" method to the Property class.
cleaned code
Merge branch 'develop' of git://github.com/sulu-cmf/sulu into feature/enhanced-structure-tagging

Conflicts:
	src/Sulu/Bundle/CoreBundle/Resources/config/content.xml
fixed tests
Merge branch 'feature/enhanced-structure-tagging' into develop