Adding updateservers to extension package

As title says.  Example of resulting zipped pack xml:
```
<?xml version="1.0" encoding="UTF-8"?>
<extension type="file" version="3.4" method="upgrade">
	<name>geshifr-FR</name>
	<version>3.4.0.1</version>
	<creationDate>08/07/2014</creationDate>
	<author>jms</author>
	<authorEmail>jms@mail.com</authorEmail>
	<authorUrl>http://asite.com</authorUrl>
	<copyright>Copyright (C) 2005 - 2014 joomla.fr et Open Source Matters. Tous droits réservés</copyright>
	<license>http://www.gnu.org/licenses/gpl-2.0.html GNU/GPL</license>
	<packager>jms</packager>
	<packagerurl>http://packager.com</packagerurl>
	<description><![CDATA[Lang files for the geshi plugin to be B/C]]></description>
	<fileset>
		<files folder="admin/fr-FR" target="administrator/language/fr-FR">
			<filename>fr-FR.plg_content_geshi.ini</filename>
			<filename>fr-FR.plg_content_geshi.sys.ini</filename>
		</files>
	</fileset>
	<updateservers>
		<server type="collection" priority="1" name="geshi plugin">http://update.joomla.org/language/geshi.xml</server>
	</updateservers>
</extension>
```