Added convenience functions to determine if a mime part is an attachment

2014-05-24  Jeffrey Stedfast  <fejj@gnome.org>

	* gmime/gmime-part.c (g_mime_part_is_attachment): New function to
	determine if a MIME part is an attachment or not.

	* gmime/gmime-disposition.c (g_mime_content_disposition_is_attachment):
	New function to determine if the Content-Disposition value matches
	"attachment".