feat(core): Introduce valdr-enabled directive
Allows to dynamically enable and disable the validation with valdr for one or multiple form elements.
closes #54, fixes #55
Merge remote-tracking branch 'origin/master'

* origin/master:
  feat(core): Introduce valdr-enabled directive Allows to dynamically enable and disable the validation with valdr for one or multiple form elements. closes #54, fixes #55

Conflicts:
	Gruntfile.js
	src/core/valdrEnabled-directive.js
	src/core/valdrEnabled-directive.spec.js
	src/core/valdrFormItem-directive.js