Added the better_errors and binding_of_caller gems

I added the [better_errors](https://github.com/charliesome/better_errors) and the [binding_of_caller](https://github.com/charliesome/better_errors) gems to the default Gemfile as they are merely drag and drop.

In short:

better_errors improves the default rails error page page (eg the user misformats a view.html.haml).

binding_of_caller lets you inspect the values of variables when the exception was thrown (better_errors supports doing this in the error page as well).