Images within Admin AJAX not handled

The Tachyon plugin currently doesn't handle any images served via admin-ajax by default. This leads to missed expectations whereby images loaded for the frontend via admin-ajax (infinite loading or load more functionality, for example) are only partially passed through Tachyon. The main image URL is _not_ Tachyoned, but the srcsets are.

One could modify this behavior with the `tachyon_disable_in_admin` filter and custom logic, but I would expect the plugin to be able to do some default differentiation and tell whether or not it should handle images in an admin-ajax context.