Fixed DecimalField arbitrary precision support

[dvarrazzo]

DecimalField has some support for non strict number of decimal places (checking here and there that decimal_places is not None but it fails in quantize() with "unsupported operand type(s) for ** or pow(): 'Decimal' and 'NoneType'".

This changeset fixes the problem and allows DecimalField to handle decimal_places=None. Tests added too.

[jpadilla]

@dvarrazzo good stuff, thanks! Anyone else has any objections/comments?

[tomchristie]

Good stuff 👍

[glarrain]

@dvarrazzo @jpadilla @tomchristie is decimal_places=None allowed then? We should update the docs to make that clear (I volunteer to do so).

On the other hand, why are args decimal_places and max_digits required then? As mentioned by @xordoquy here, those that are not kwargs to __init__ are supposed to be required (because they can not be set to a default value).