New definitions for various libraries I use.

Definitions added:

 - libxml2 is a native (C-based), open source, cross-platform XML parser.
 - win32ole is part of the MRI standard library on the Windows platform.
 - GLib2 is a native (C-based), open source, cross-platform, general infrastructure library used heavily by GNOME and programs loosely or tightly related to it (GTK, etc.)
 - GTK3 is a native (C-based), open source, cross-platform GUI toolkit.
 - RubyTree is a pure Ruby library for constructing n-ary trees of arbitrary nodes of data, and performing various algorithms on them.
 - alm-rest-api is a pure Ruby library implementing parts of the client-side REST protocol and object model for interfacing with the product "Application Lifecycle Management" by Hewlett-Packard Development Company, L.P. This is the only gem in this list that is in any way related to proprietary software; the rest are free and open source software.

Feel free to cherry pick if you only want some of these. I split them up into separate commits. I included GLib with GTK in one commit, because it's actually rather difficult to write a decent program in GTK without referencing anything from GLib, so if you're pulling one, you might as well pull the other.

**NOTE:** All of these definitions are for gems that are available for public download from rubygems.org. All of the gems themselves are free and open source software, even if one of them (alm-rest-api) can only be used if you have access to a server running specific proprietary software.