Add dummy middleware that removes ESI tags from the DOM

[crccheck]

Closes #14

Functionality is complete, but missing changes to support.

If 'armstrong.esi.middleware.DummyEsiMiddleware' is in your MIDDLEWARE_CLASSES, esis will be rendered as comments instead of esi tags:

    <!--esi:include src="/esi/accounts/analytics/" /-->

instead of the rendered esi or the original esi tag:

    <esi:include src="/esi/accounts/analytics/" />

RFC

The thought presently occurs that perhaps this should be NullEsiMiddleware instead of DummyEsiMiddleware.

TODO

• make sure it works with esi comments (see section 3.7 of http://www.w3.org/TR/esi-lang)
• Add tests
• Update documentation

[crccheck]

bump. issue #14 is driving me crazy.