Show backtrace location

I use r() a lot when I debug, but often I forget where exactly I used it. This adds the file and line number on which the r/rt.

![screen shot 2013-12-16 at 7 22 25 pm](https://f.cloud.github.com/assets/103370/1756721/b5c7e3aa-6676-11e3-8354-facb9ecf4d99.png)