Clean up sample text and links
Add SIMD sample description
Update links for new crosswalk-samples version
Update unwieldy paragraph
Add SIMD sample icon
Add note about minimum version of Crosswalk + arch required for SIMD

Only works on beta Crosswalk on x86.
Add download links to all sample-specific pages
Merge pull request #97 from townxelliot/simd-sample-app-1237

SIMD sample app