build: Do not hardcode path to libtool

Some OSes ship with their own libtool and Autotools can use the
LIBTOOL env var to specify the path to libtool. So make it possible to
use this variable as the libtool path for the introspection scanner as
well. This fixes the build on at least OpenBSD.

https://bugzilla.gnome.org/show_bug.cgi?id=726570
Updated Khmer translation
build: Clean up uses of INCLUDES and _CPPFLAGS

INCLUDES is deprecated in favour of AM_CPPFLAGS and per-target
_CPPFLAGS. Since AM_CPPFLAGS is ignored in preference to a per-target
_CPPFLAGS variable, we should define the latter only when it is needed.

Fixes: https://bugzilla.gnome.org/726570
build: Use Makefile.introspection

Fixes: https://bugzilla.gnome.org/726570
Updated German translation
live: Remove chat support

Microsoft has been planning to drop Windows Live Messenger in favour of
Skype for quite some time now. The XMPP gateway that this provider used
does not work anymore. However, you can continue using the proprietary
MSN protocol by selecting "Others -> Windows Live (MSN)".

Fixes: https://bugzilla.gnome.org/728787
live: Don't leak the GoaDocuments

Fixes: https://bugzilla.gnome.org/728787