`TimeSeries.plot` ignores `color` kwarg
I know this looks like a duplicate of #1636, but even with pandas 0.8.1, I don't get the expected plotting behaviour:

```
In [1]: from numpy import arange

In [2]: import matplotlib.pyplot as plt

In [3]: import pandas

In [4]: pandas.__version__
Out[4]: '0.8.1'

In [5]: plt.figure()
Out[5]: <matplotlib.figure.Figure at 0x33afd50>

In [6]: plt.plot(arange(12), arange(12), color='green')     # this line is green
Out[6]: [<matplotlib.lines.Line2D at 0x337c150>]

In [7]: pandas.Series(arange(12) + 1).plot(color='green')   # this line is blue
Out[7]: <matplotlib.axes.AxesSubplot at 0x33c80d0>
```
