Add Christmas logo

Logo is from Anna Bujak. See
http://www.google-melange.com/gci/task/view/google/gci2011/7237320

The png removed the white background from a mask. A better job should be done
using Inkscape.
Make the SymPy Christmas logo smaller
Revert "Make the SymPy Christmas logo smaller"

This reverts commit 5ae37b04261c2073362510b7c71770277339fe81.
Fix christmas logo
Make Christmas logo more square
Revert "Make Christmas logo more square"

This reverts commit 77733f5e091b498d9be62e3d43df4262a375e0ac.
Use better aspect ratio for Christmas logo
Logo a little bigger
Merge pull request #3 from lidavidm/fix_sidebar

Fix Sphinx shell resizing
Add images and styling for new features page