FIX: Use asfarray on data and indep vars if needed.

Revision based on discussion in #126:

* If data/vars implement the array interface (`__array__`), do not modify them.
* If not, convert them to a float array using `np.asfarray`.

This supports lists, tuples, and any iterables, and it converts `None` to `np.nan`, which will then be treated as missing data. This does not disrupt support for Series or other array-like objects.