Misc Model improvements...

* API: ``N=Parameter(value=7)`` is equivalent to ``N=Parameter(name='N', val
* CLN/PERF: Build residual only once, on instantiation.
* ENH: Allow models with no independent_vars.
* ENH: Support weighted fits.
* BUG: Copy parameters on input.
* ENH: Handle missing data.
* BUG: Accept pandas.Series equivalently to numpy.ndarray.