Escape project name in notifications to Slack

Hi bugsnag team,

We need to escape several characters to notify Slack. Please refer to the [Slack message formatting](https://api.slack.com/docs/formatting).

I also attached what happens without this fix below.

![screen shot 2014-08-16 at 11 15 29 pm](https://cloud.githubusercontent.com/assets/1162120/3942065/6ffe07c8-2552-11e4-97c3-8c016dd1247e.png)