Add auto-language detection to Tweetbutton

Attached is an unsophisticated, but working patch (against 6.14) that adds an "Automatic" option to the "Language" select at /admin/settings/tweetbutton.

If the contents of the tweetbutton_language variable is found to be 'auto', the module runs global $language and provides the contents of $language->language to tweetbutton_get_attributes().

If you think you'd like to include it but that it needs modification, please let me know and I'll make whatever changes you think necessary.