Multiple calls to .plot(): colors not correctly cycled
Versions:
Pandas 0.14.1
Matplotlib: 1.4.0

Reproduce:
Run:

``` python
import matplotlib.pyplot as plt
import numpy as np
import pandas as pd

for i in range(20):
    pd.Series(np.random.rand(20)).plot(label=i)
plt.legend()
```

Output:
![index](https://cloud.githubusercontent.com/assets/674200/4233081/13884414-39a6-11e4-9bc3-79586b2b2906.png)
You can see that the colors are cycled once and then only blue (i.e. the first color) is being used.

Expected output:
![index2](https://cloud.githubusercontent.com/assets/674200/4233099/3e7a0d38-39a6-11e4-8943-68b3405a996a.png)
You can see that colors are being cycled correctly.

The expected output was produced using matplotlib's `plot()` command. But note that if `Series.plot()` was run once in the interpreter, matplotlib's `plot()` command produces the same errenous behavior (only plotting blue after the first cycle). It thus appears that `Series.plot()` sets some global matplotlib state that causes this.
