Add 'size' param to TimberImage::get_src to get image src for a predefined WP size.

Uses image_downsize, so plugins modifying image resizing can pass custom url
Merge branch 'image-sizes' of github.com:mgmartel/timber into mgmartel-image-sizes

* 'image-sizes' of github.com:mgmartel/timber:
  Add 'size' param to TimberImage::get_src to get image src for a predefined WP size.
Merge branch 'dev' of github.com:jarednova/timber into dev

* 'dev' of github.com:jarednova/timber:
  Set TimberLoader::get_cache default return to false, to force cache refresh when cache mode is 'none'