Change to the approach fixtures take

This release fixes a number of issues with the previous release img-fixture element. The key change however is how we handle fixtures in general. It is now expect that fixtures will return their entire HTML body not just the internals, for example a Fixture > Text element previously of the form:

<p>
    Foo
</p>

Would have returned:

>>> Pre 1.3.0

<- Foo

>>> 1.3.0 onwards

<p>
    Foo
</p>

For the same reason a new release will follow shortly for ContentTools which supports this new approach and which allows fixtures to be inspected in the editor.