[MEDIUM] add support for conditional HTTP redirection

[MEDIUM] add support for conditional HTTP redirection

A new "redirect" keyword adds the ability to send an HTTP 301/302/303
redirection to either an absolute location or to a prefix followed by
the original URI. The redirection is conditionned by ACL rules, so it
becomes very easy to move parts of a site to another site using this.

This work was almost entirely done at Exceliance by Emeric Brun.

A test-case has been added in the tests/ directory.