removing String to Array conversion while invalidate
Merge branch 'develop' into feature/skyrocketingSpeedup
Merge branch 'develop' into feature/skyrocketingSpeedup
Merge branch 'feature/skyrocketingSpeedup' of github.com:AKSW/Erfurt into next

Conflicts:
	library/Erfurt/Cache/Frontend/QueryCache.php
revert new approach of TitleHelper
remove obsolete PropertyHelperTest.php
improve MemoryModel

improve some tests

improve class itself:
- by use Erfurt_Uri::check instead of checking only for
  null and is_string
- simply stuff
- refactor comments and code