Add the git protocol to the django-haystack dependency link

Without it, it is a malformed VCS url
Add migration help to customise models docs
Docs: Remove duplicate OSCAR_IMAGE_FOLDER entry
Add note to docs for customising classes of products.
Improve sandbox installation process

* Remove profiling middleware from default setup
* Document dependency on libjpeg
Adjust post-user-create receiver to check database state

Need to avoid situation where this signal gets raised by syncdb and the
database isn't in the correct state.

Fixes #475
Document dependency on compressor in default templates

See https://groups.google.com/forum/#!topic/django-oscar/-F1rEUX-RZA
Update installation doc

* Use `code-block` to see if that renders differently
Fix URL for category update view in dashboard.

Detail view and update view had the same URL,
update view is now /pk/update/ as elsewhere.