Return status codes and headers, not just mimetypes

Plumbs the way toward returning additional headers (and non-200 status codes)
from deeper within.
Move getTile onto Layer
Fix up tests
Borrowed httplib.responses to look up status code reason strings
Changed instances (all?) of headers on WSGI path to use wsgiref.headers.Headers
Changed instances (all?) of headers on CGI and mod_python paths to use wsgiref.headers.Headers
Centralized CORS and cache header handling to requestHandler()
Removed all uses of TheTileIsInAnotherCastle and merged layer redirects behavior into requestHandler() with new script_name function parameter
Added Core.TheTileLeftANote properties content and emit_content_type and streamlined exception handling
Removed default content type from all responses
Removed makeHeaders
Reinstated TileStache.getTile()