Allow, render legend attributes on fieldsets

A form that subclasses BetterForm should have the ability to declare legend names for each fieldset. This PR allows users to set a value for an optional legend key within the Meta fieldsets list.
(see above) ... and `self.legent = self.fieldset.legend`. Or even:
```python
@property
def legend(self):
  return self.fieldset.legend
```

What do you think?