Add a policies collection to brokers and include it in the broker details
* app.rb: turn off Sinatra's pretty exceptions

In development mode, Sinatra by default generates a detailed error
page. Since Razor is mostly used from the command line, these detailed
errors are unintelligible on a console and only lead to useless spew.
* Rakefile (db:nuke): also drop the power_state type

Otherwise, recreating a database doesn't work; this is all annoyingly
fragile and we need a better strategy for recreating the database, ideally
without resorting to drop + create database since that is highly annoying
in development.
Add a delete-broker command

Fixes https://tickets.puppetlabs.com/browse/RAZOR-87