MKT-696 -- Mobile MVP Homepage

## TODO
- [ ] Insert graph background image
- [x] Insert company logo SVGs
- [ ] Insert Illustrations in reasons section
- [ ] CTA Button redirect to pricing page
- [ ] Change font in company logo section
- [ ] Test on IE

### Things I don't like:
1) The Reasons section (second section) on tablet. One column stacked boxes doesn't look right. I'm open to suggestions.
2) "Deliver your best customer experiences at every touchpoint on the web and native mobile apps" header on iPhone wraps the screen 3 times. Looks cluttered.
3) I had to put an id on [this line](https://github.com/optimizely/marketing-website/compare/ssmith/homepage#diff-f0209f6a926f957cf6b8f1a2738265e5R21) in order to take off the margin because the class "four" was leaving one box on the next row.


@stewsmith use a `<section>` tag instead of a div for individual sections of an html document