sfc: split donation types with headers

Commit da3dfc0 (drop Google Checkout link from SFC donation
page, 2013-11-01) reformatted the list donation options into
a list. It probably should have split "Wire" into its
category. Later, a5866e6 (sfc: mention that we can take
stock donations, 2013-12-06) made this even worse as we
added a new type that should get its own heading.

This patch adds in separate headings for "Check", "Wire",
and "Stock".