Add update notifications

closes #1464

- adds opt-out via updateCheck:false in config.js
- update check is done on admin index, but doesn't interfere with rendering
- adds update check module, which gets the usage data, makes the request and handles the response
- adds two new settings to default-settings, one for next check time, and one for whether to show the notification
- adds a new rejectError method to errorHandling
- adds a new helper for displaying the notification

Notification look like old style notifications:

![notification](http://puu.sh/68wi4.png)

Example data:

```
{"ghost_version":"0.3.3","node_version":"0.10.23","env":"development","database_
type":"sqlite3","email_transport":"Mailgun","blog_id":"095ac57dd53d57f3fc7997131
69589a4","theme":"casper","apps":"example-app, import-test","post_count":19,"use
r_count":1,"blog_created_at":1385420204,"npm_version":"1.3.17"}
```