[himura467]

Closes #20

• Adds Nack::URLMap, built from a Hash of references to apps. A reference is an absolute-path reference (/path) or a network-path reference (//host/path); one without a leading slash, with an empty host, or with a port raises ArgumentError.
• call moves the matched prefix from path_info to script_name, calls the app, and restores both on the way out, also when the app raises.
• A prefix matches at a segment boundary, so /a matches /a and /a/b but not /ab. A mount for the request's host wins, and among the rest the longest prefix wins. Hosts compare case-insensitively and ignore the request's port.
• A request whose path is nil (authority-form CONNECT) or * (asterisk-form OPTIONS) matches no prefix. Anything unmatched gets a 404.
• Paths are compared as received, without percent-decoding or dot-segment removal.