Quoted target support

Grunt will accept quoted target names, which is useful if you want hyphens in target names, for example:
```javascript
targethtml: {
  'dev-test': {
    files: {
      'tmp/dev.html': 'test/fixtures/index.html'
    }
  }
}
```

This pull request is to add support for the *literal* expression.type to the *conditionalParser* function, in order to make quoted target names work again. It stopped working after the introduction of the conditional support/esprima.

This pull request includes the change itself (which is quite small), plus a few tests and more verbose error messages that helped during my troubleshooting.

I've been using this for a while now and it works for me.