Move the Picturefill_WP class to its own file.
re: issue #7
Define version constant.
Use a singleton design pattern
Rename `php/` to `inc/`
Using a `php/` directory seemed natural, but `inc/` seems to be much more common. Hopefully this will add clarity for developers.
Implement a basic template system to separate out logic from presentation.
Add back in `min-size` setting
Create a `model` object for each image and setup the image data on `__construct`
Move the `set_image_sizes` to the model class.
Add the `__clone` and `__wakeup` magic methods
for a more complete singleton class
Update version information