Review the need to monkey patch on modern Django

[jezdez]

django/django@af64429 adds support for __html__

[jsocol]

As long as we support 1.4 ESR, 1.5, or 1.6, we'll still need it, but we can update the docs to point out that it's unnecessary for new-enough Django.

[jsocol]

It turns out we can't necessarily drop it, see #61. Django templates support the __html__ proto but some form classes seem to not implement it. Needs further research.