Add better_errors with binding_of_caller

Shows better stack trace with live shell to interact
with environment when error occurs.

![before](http://cl.ly/image/2r3u370i1p2M/Screen%20Shot%202014-10-21%20at%209.51.01%20PM.png)

becomes...

![after](http://cl.ly/image/332H1D3w2x0Z/Screen%20Shot%202014-10-21%20at%209.42.35%20PM.png)