Make the website more SEO friendly

As a fun developper I want to play new games and find easily MemoryOverflow.

**Technical criteria:**
- [ ] Add all bests meta tags for best SEO
- [ ] Add more descriptive paragraphs in the homepage
- [ ] Add the Google Custom Search (osd.xml)
- [ ] Add Facebook meta tags
- [ ] Add Google+ meta tags
- [ ] Add Google+ author (create Google+ account)
- [ ] Add a sitemap
