Change backgrounds for Sponsors and Team sections

According to the PSD
Fix Sponsors layout according to PSD
Change `Partners` for `Supporters`
Add RedHat as a Gold Sponsors
Center logos to prettify the display
Fix RedHat URL
Add ElasticSearch as Silver Sponsor
Make some sponsors logos bigger
Merge branch 'gh-pages' of github.com:rupyconf/13.rupy.eu into gh-pages
Integrate last CSS and HTML model for Sponsors
Add di24 and pyvec as partners
Merge pull request #23 from AdrienGiboire/gh-pages

Add latest sponsors