Added the Installer module to Horde_Element. It allows to install a PEAR based Horde...

Added the Installer module to Horde_Element. It allows to install a PEAR based Horde framework package from the repository tree, including its dependencies. Horde dependencies will also be pulled from the current tree, remote dependencies will be downloaded.

This should now allow a continuous integration (CI) setup that tests
the PEAR based Horde framework packages including their
dependencies. While we will support PEAR based releases for Horde4 I
assume from past experiences that the focus will remain on the
applications. As those rely on a fully installed framework it will be
quite easy to overlook PEAR dependency problems. So I feel it is
necessary to include the full installation method via PEAR into the CI
in order to spot problems early.

The general idea of the Horde_Element package is to provide tools for
managing Horde "Elements" which I consider to be PEAR based packages
from the framework (and maybe later also the applications). While I'm
currently focusing on the basics I think it would make sense to later
add methods that allow publishing the elements better than via
pear.horde.org.