Partial refactoring of the builder
Merge branch 'release-2.0' into builder-refactor
#844: Add Validator Service and libraries
Merge branch 'release-2.0' into 844

Conflicts:
	composer.lock
Fix #865: $element should be $item
#844: Continue work on the Builder refactoring

- Moved the Descriptor initialization to a ServiceProvider
- Added FileDescriptor
- Implemented all functionality from Reflector Builder into Assemblers
- Refactored Parser to use new builder.
#844: Update composer after merge
#844: Add validation using the Symfony Validator Component
Finish implementation of #844
Merge pull request #845 from mvriel/844

[WIP] #844: Refactor Builder component