Add check for slug when editing page (fixes #119)
Improve JS behavior when editing slug (fixes #132)
Fix bug introduced by a1c9224

The a1c9224 commit allowed page editing only when their slug had been
changed!
Add support for setting global image compression..

quality. Uses an IMAGE_COMPRESSION_QUALITY in the settings.py, which, if
set, will set a quality attribute in the image processing backends. Each
image processing backend then can use the quality attribute in its "save"
method.

This has been implemented in both Pillow and Wand backends. It has been
tested with image that support this setting (JPG) and images that do not
support it (GIF, PNG) and the behavior is consistent.

This is an implementation for #163.
Change slug cleaning to only happen on blur, not every keypress - fixes #132

a854c03f443083a1471dd066dece92527dbb7c7c improves the behaviour a lot, but it's
still fairly nasty for a text field to be rewriting text as you type it. It's
also unnecessary - you have to be pretty determined to submit the form without
triggering the blur event on the field, and if you do, all that happens is that
you fail server-side validation.
a spot of PEP8 cleanup on pillow.py / wand.py
Set a default IMAGE_COMPRESSION_QUALITY of 85 as per @tomdyson's suggestion, and simplify image backend now that it doesn't have to check for the presence of a quality setting