Setting a `main` field that makes sense.

The Gruntfile is just a dev dep of this project - it makes no sense to have it as a `main`. Conversely, setting `lib/generate-message.js` to be `main` allows someone to re-use this task's business logic via `require('grunt-attention')`.
Merge pull request #1 from NickHeiner/master

Setting a `main` field that makes sense.