Add RightSupport::Rack::Runtime middleware for Rack

This middleware allows reporting of the runtime of all Rack apps at and below this middleware in the stack via an 'X-Runtime[-<name>]' header in HTTP Response.

Usage:
use RightSupport::Rack::Runtime          # => 'X-Runtime' header added
use RightSupport::Rack::Runtime, "Total" # => 'X-Runtime-Total' header added