Clean up: Refactor JS includes, use SVG Logo (with fallback)

While writing the small PR earlier today to add the rustbyexample.com link (#72), I saw some pretty easy optimization opportunities.

This PR contains:

- Moving some code to separate files in `_includes` to make it more readable
- Use the SVG logo where possible (else fall back to the PNG one used previously)
- Optimize the images includes using ImageOptim
- Minify and embed the custom JavaScript files
- Load the ACE editor files asynchronously

There is no new functionality added in this PR. I have tested the site using Chrome on OS X and IE9 on WIn7 (browser stack, only shortly, though). I would welcome it if some people on other OS' could confirm it still works.