MAINT: remove duplicate Plotly entry from topical software page.

See PR 39.
ENH: add donations page to the scipylib area of scipy.org
MAINT: add SourceForge to non-financial support list.
MAINT: add GSOC to the donor list.
MAINT: fix grammar (addresses review comment of @josef-pkt).
MAINT: finish donations page, add PayPal button.
MAINT: update date on donations page to today.