Major refactor of module

[kinglozzer]

• Upgrade to picturefill v3.0.2
• Change YAML format to allow more than 2 arguments to be passed to image processing methods
• Change YAML format for "sizes" to :
• Rename "sizes" to "arguments" and "default_size" to "default_arguments" in config
• Namespace extension class

[kinglozzer]

I’ve made the changes suggested in #14, but removed the HtmlEditorField/srcset integration and put it in its own module - upon reflection I felt it should be a separate module to this (feel free to add it to suggested modules in composer.json if you like).

[camspiers]

cc @stecman

[stecman]

@kinglozzer can you change the namespace to Heyday\ResponsiveImages? It's more inline with what we've got elsewhere. Happy to merge after that 👍

[kinglozzer]

@stecman Done :)