Introduces Textpattern_Http_Request.

Collects all the HTTP request related stuff to a single
usable package. Contains:

* Content negotiation.
* Extended proxy support (HTTPS).
* Header extraction, and reading raw data.
* Absolute URLs, ports, hostnames, DNS resolving.
* Reading query parameters and cookies.
* Getting complete cleaned data maps in addition to values.

This class will allow creating multilingual (or otherwise
variable) interfaces that use content negotiation, and
offers some cross-server and proxy abstraction. It
collects all the very usable, but one-off snippets from
Textpattern source to a reusable package.

Still probably needs some sanitizer snippets that are
scattered in Textpattern's source and multi-query-string
extraction ala psa() and gpsa(). And of course, adding
this in.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5642 2fea6a4d-a838-0410-917b-93a53c48e9d2