Adjusting specialspaceless to not break intended spaces

Fixes #250
Added {{ form.media }} to form rendering

This should tackle issue #263
Changing how KeepContext works, added extra_context kwarg to render

This breaks backwards compatibility in two ways:

* People having custom layout objects, that manipulate context itself
will have the possibility to suffer side effects. However this context
iteration was too expensive for some people causing performance issues.
* People with custom layout objects without **kwargs in their render
method won't be able to receive `extra_context` or pass it where needed.

This should fix #264
Fixing bootstrap3 checkboxes alignment for all device sizes

Fixes #225 and #267

Thanks to @pySilver for proposing a patch for `field.html` template.
Updating CHANGELOG and CONTRIBUTORS