Remove wagtail dependency

[specialunderwear]

Wagtail has a very nice querycompiler implementation that allows orm queries to be compiled to elasticsearch queries. Instead of duplicating code from another project by copying, I just added the dependency to wagtail. That works just fine if you always use wagtail together with oscar. It would be nice if after optimizing and contributing the code in this package to wagtail, we can convince the wagtail maintainers the querycompiler can server a greater good then just wagtail and extract it to a separate package.