BUG: fix handling of color argument for variety of plotting functions
parallel_coordinates
- fix reordering of class column (from set) causing possible color/class
  mismatch
- deprecated use of argument colors in favor of color

radviz
- fix reordering of class column (from set) causing possible color/class
  mismatch
- added explicit color keyword argument (avoids multiple values 'color' being
  passed to plotting method)

andrews_curves
- added explicit color keyword argument (avoids multiple values 'color' being
  passed to plotting method)

To recreate the bug:

``` python

import pandas as pd
from pandas.tools.plotting import parallel_coordinates, radviz, andrews_curves

x = pd.DataFrame([[1,2,3,'b'],
                  [2,2,1,'g'],
                  [3,3,1,'r']], columns=[0,1,2,'name'])

parallel_coordinates(x, 'name', colors=['b','g','r'])

# TypeError from scatter
radviz(x, 'name', color=['b','g','r'])

# TypeError from plot
andrews_curves(x, 'name', color=['b','g','r'])

```

parallel_coordinates before:
![ppl_old](https://cloud.githubusercontent.com/assets/3686522/2791463/28c84c9a-cbc7-11e3-9099-9e064d31e995.png)

parallel_coordinates after:
![ppl_new](https://cloud.githubusercontent.com/assets/3686522/2791462/28c73e0e-cbc7-11e3-804e-73dc43cbd6ca.png)
