Added 'double arrow' (DArrow) to patches.BoxStyle.
Added image test for arrow patches.

All bbox's in patches.BoxStyle are included in the test image.
Update patches doc and example.

Added Circle to annotations_guide.rst. Moved DArrow up so that entries
are sorted. Made fancybox_demo2.py outputs sorted box demo (so that
ordering matches annotations_guide). Increased spacing between boxes in
fancybox_demo2 to prevent overlap of images.
Merge pull request #3194 from lkilcher/annotate_bbox_DArrow

Annotate bbox darrow