Added AlphaLab and Stitch Fix logos & descriptions

Added logos & descriptions for AlphaLab and Stitch Fix to the sponsors
page and the sponsors section of the html.

Also made the following changes to sponsorship.html css:

* Updated the styles same font as the rest of the site. It was
  using Georgia instead of Lusitana.

* 1999 called and wanted their <center> tags back, so they were removed
  and replaced with text-align:center

* Increased the vertical margins on the logos and text. It was looking
  a little cramped.