Mirror salt.utils.pydsl.StateDeclaration.__getattr__ as __getitem__

This is helpful e.g. in top.sls files where non-keyword like attributes
of the state('base') object often get accessed. This little patch allows
to write things like:

  state('base')['*'].foo
Merge pull request #4250 from spacedentist/develop

Mirror salt.utils.pydsl.StateDeclaration.__getattr__ as __getitem__