Update Android getting started instructions for new folder layout

There is no longer an xwalk_app_template.tar.gz file to unpack
after downloading Crosswalk Android (the top-level directory
after you've unpacked is the app template).

Fix the instructions, removing all references to this second
unpacking stage.
Merge pull request #96 from townxelliot/new-dl-structure

Update Android getting started instructions for new folder layout
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