New error handling for Phoenix

This pull request changes Phoenix to use Plug 0.8.3 which contains the new `Plug.Debugger` and also migrates to the new error handling using a view as defined in #482.

It also changes Phoenix code reloader to proxy the output from the terminal to the screen in case there is a failure when compiling code.

Here is a screenshot of the new error pages:

![Plug.Debugger in action](http://i.imgur.com/A0oM7hm.png)