Article section (.//article-categories/subj-group[@subj-group-type="heading"]) validation
Adjustments #23
Merge branch 'master' of git://github.com/scieloorg/balaio into 23

Conflicts:
	balaio/tests/test_validator.py
	balaio/validator.py
Issue #23
Merge branch 'master' of git://github.com/scieloorg/balaio into 23
Inserção do ArticleSectionValidationPipe no Pipeline
Merge pull request #68 from robertatakenaka/23

Validação da seção do artigo (Issue #23)