Added Facebook's XHP library to www codebase.
Beginning of XHP implementation.

Implemented base structures that are used to render the website's
frame. Converted everything which was in views/header views/footer
and views/sitewide into xhp objects and added a method in
www/utilities which composes an <ia:page> object with the site's header
sidebar and footer in place. Views can then append a <ia:content> child
containing the page specific content and render the entire page. All
current views still use the legacy execute_view method, which renders
the main content in a buffer, adds it to a <ia:content> and uses
<ia:page> for the site's frame.

XHP objects are places in www/xhp/{namespace}/{object}.php. The
standard XHP library provided by Facebook is added in www/xhp/base.
Reusable widgets should use the ui namespace. This is where most
of the features provided by format_* can be added. Other widgets,
such as the website's sidebar, which is not reusable should use the
ia namespace.

As a coding style note: indent is still 4 spaces for normal code.
Elements inside a xhp object are indented using only 2 spaces, which
makes the code more readable.

There are some hacks in this patch, but fixing them would require
some big changes and this diff is already pretty huge.