Integrate notifications into core

This does 2 things:

* Make the notifications package a bundled package
* Add notifications (Errors) for issues when loading files (config, keymap, snippets)

There is an issue on error where two messages are displayed. 

![screen shot 2014-11-24 at 5 21 01 pm](https://cloud.githubusercontent.com/assets/69169/5176450/4c2aa1bc-73fe-11e4-9c40-a6602b794f7e.png)

This is caused by https://github.com/atom/node-pathwatcher/issues/50