Replaced lxml with html5lib
Fixed test which broke under html5lib

The test used HTML which wasn't valid HTML5. html5lib corrects these errors whereas lxml didn't so the result of the test is different.
Merge branch 'html5lib' of https://github.com/kaedroho/wagtail into kaedroho-html5lib
Update test_image_embed_is_rewritten to work on valid HTML input, to make it a more meaningful / representative test
add html5lib item to changelog + release notes
Merge branch 'kaedroho-html5lib'
Merge branch 'master' of github.com:torchbox/wagtail