Add rack-parser to Gemfile

We'll use it to parse request bodies (which may contain JSON)
Don't accept empty domain/hostname.
Better responses when exceptions are raised...

... the Commander::App should be able to respond with better return
codes/messages when exceptions occur. Here we introduce support for
handling a few well-known exception types, and returning appropriate
codes for each (along with a body).

Ideally, this could also be done in a middleware object, but we'll
punt on it for now.
Use Rack::Parser middleware.

Use Rack::Builder to create a stacked app (with commander at its
heart). Pass that on to the http server. This allows us to use
parsing middleware to accept json in the body.
Add ruby-dbus to list of dependencies (Gemfile)

We'll use this to communicate with upstart.