Add nbviewer random thumbnails.
Fix doc links, include all older release links as well and reorg page.
Fix link to current stable version (1.1.0)
Create separate sponsors page with Sloan info.
Fix some language in install page.
Fix centering of donate button.
Add links.txt file so we can reuse links more easily.
Merge pull request #35 from fperez/updates-various

Various updates: new sponsors page (Sloan), docs links, small fixes.