Importer descriptions i18n

[SergeyBiryukov]

At first I've tried to create a ticket on ​http://plugins.trac.wordpress.org/, but none of the importers are present in the Component dropdown. So I've decided to try it here.

As ​noted by Rasheed Bydousi on WP Polyglots, importer desciptions are displayed in English. The problem is that register_importer() function is called before the language file is loaded.

I've created patches for all the importers. I've also changed init action to admin_init, which seems to have more sense in this case.