Use Python path functions in image styles

[Wtower]

The image_style function of utils/media.py is responsible for generating image styles.

It currently utilises the linux file name conventions explicitly, which makes this function unavailable to Windows.

Also, the way that the paths are constructed is not Pythonic and causes trouble to the tests in #44 as the media root there is different than as assumed in most projects.

Method:

• Instead of passing the url string pass the ImageField object itself in order to use the file property (instead of the url property that was used in templates).
• Then return the appropriate url.
• Also change the templatetags to remove the @stringfilter and the variable names to reflect the change.
• Consequently, the image_style_form template tag is now obsolete. This was totally unnecessary anyway, the initial reasoning was that form imagefields are different objects but the ImageField object can be obtained from them.
• Change the block_static.html example template from <img src="{{ img.image.url|image_style:'large' }}"> to <img src="{{ img.image|image_style:'large' }}">
• Change the templates stacked.html that displays thumbnails in node editing from <img src="{{ inline_admin_form.form.image.value|image_style_form:'thumbnail' }}"> to <img src="{{ inline_admin_form.form.image.value|image_style:'thumbnail' }}"> (the value property is the underlying model ImageField in the form ImageField).
• Update CHANGELOG and make note that when updating 9cms, this is requires manual updates to all templates using images styles.