[add] negative lags

[kwgoodman]

Add ability to specify a negative lag.

lar.lag(1) currently works; but lar.lag(-1) raises a ValueError:

larry/la/deflarry.pyc in lag(self, nlag, axis)
   2604 raise IndexError, 'axis cannot be None.'
   2605 if nlag < 0:
-> 2606 raise ValueError, 'nlag cannot be negative'
   2607 y = self.copy()
   2608 y.label[axis] = y.label[axis][nlag:]

ValueError: nlag cannot be negative