Use media attachment types as contexts

This is for issue #260.

The media connector now references the media type as the context,
instead of simply repeating the connector (‘Media’).

This will not effect previous Media connector entries, which will still
show the context as 'Media'. Is it worthwhile adding to the 
WP_Stream_Install->update() function to remedy this?

The media types are borrowed from wp_ext2type() in
wp-includes/functions.php.

(also - this is my first Stream pull request!)