Updated go.text/language package sources

This package's repo uses Mercurial, so we can't make it a submodule.
Copied in the contents of hg revision b1a7e9967ab3:

changeset:   41:b1a7e9967ab3
user:        Nigel Tao <nigeltao@golang.org>
date:        Fri Sep 27 18:12:46 2013 +1000
summary:     go.text/encoding: add Replacement and XUserDefined encodings.

Note: There is one newer revision (40d3e0c98d89) but it breaks
compatibility with Go 1.1 so I didn't update to it.