Translatable message and button in activity stream and single media view.

I use the buddypress activity stream and noticed that it was not possible to translate some items.
First, the message "xy added a photo" already went through the translation function, but did not show up in rtmedia.po.
Second, the upload button that occurs when using the "what's new" field to upload media should be translatable.

Also, 'Comments'-title, comment author and date in single media view are now translatable.