Wrote a function returning an image element with srcset and sizes attributes. See this Gist for the function and this Gist for an example of the function in use (would need to be within the WP loop).

Wanted to give a front-end dev like Sam the ability to define the important bits, including the default image size for the src attribute, the media queries for the sizes attribute, and classes for the img element if necessary.

Edit 23 Jan 2019
This isn’t necessary anymore, responsive images have been part of WP core since v4.4. They’re implemented on wp_content automatically via a filter. Use their related functions if you need to do something custom. Responsive images are behaving a little erratically on my site though, so will have to take a look at why that might be happening.