Simple tag refactor

Simple tag is easier!

At this point we don't need to support older Django's that didn't have `simple_tag()` and `takes_context` was a new argument in Django 1.3. Speaking of 1.3, three tests break. The commit message explains it, but I'll mention it again:

`simple_tag()` cannot handle filters in Django 1.3 so this causes 3 tests to fail. In reality though, the only scenario effected is `{% render_list list_var|filter %}`. That *previously* worked whereas filtering the template arguments to List/Next/Remainder never worked. (It was fixed in ebe183dc466cac52eb58c00f97bc28996f65ed15 but that was never in a public release.)

I'm okay letting it slide. It's an edge case and only breaks in 1.3.