Add support for sending code snippets with stacktrace

![screenshot 2014-12-15 17 59 00](https://cloud.githubusercontent.com/assets/104009/5447536/466d054e-8484-11e4-8712-3f8154071e54.png)

Needs syntax highlighting support in bugsnag/bugsnag-website
@loopj We'll need to handle the case where the file can't be read. Can we just wrap this whole thing in rescue nil?