Changing properties used on `this` in helpers

Changing `this.root` to `this.app`
Changing `this.context` to `this.ctx`
These changes only affect helpers that used `this.root` or `this.context`.

@jonschlinkert doing a PR for this since there are some helpers in `verb` and `assemble` that need to be updated. Also, the new [`helper-license`](https://github.com/helpers/helper-license/blob/master/index.js#L24) will need to be updated.