ARCH: Intermodal::Base should inherit from Engine

[Ho-Sheng Hsiao]

• This will allow us to write a per-API configuration
• This may allow our controller DSL to auto-generate routes
• Instead of the bad way of loading an API (explicit require in environments/application.rb), we can simply mount the entire API in routes (or have it auto-mounted)
• This might solve the problem with autoloading