Fixes memory leak on generic Doctrine Writer

The writer uses now the ProductCacheClearer
Add notification entities, repository and specs
Merge branch '1.2' into fix/PIM-3121

Conflicts:
	CHANGELOG.md
PIM-3151: Add notification factory, manager and specs for it
Merge branch 'master' into fix/PIM-3121

Conflicts:
	CHANGELOG.md
Merge pull request #1433 from akeneo/fix/PIM-3121

Fixes memory leak on generic Doctrine Writer
Fixes CHANGELOG
Set version number
Merge branch '1.2'
Fixed writer bugging with attribute group import
Merge branch '1.2' of git://github.com/akeneo/pim-community-dev into 1.2
WIP first POC on product PDF generation
WIP add a compiler pass and tagged generators
Fixed bug with updating of associations
WIP full template generation with tagged services
WIP fix comments and licenses throughout recently added files
Fixed print_r of entities causing out of memory errors
Add missing queries in upgrade doc
Add missing queries in upgrade doc
WIP html template