BUG: Fixes color selection in andrews_curve
Fixed bug introduced in b5265a5b84ae1aeaf5fff88f1f9e5872a9b404e6 which change from picking a single random color for the class to picking a random color for each line.

This returns the color selection to a per-set basis.  

See http://stackoverflow.com/questions/19667209/colors-in-andrews-curves

Please post an image of output you get from this.
 (and nice to have reproducible example, even if it's just copying from SO question)
 ```
import pandas as pd
import numpy as np

df = pd.DataFrame(np.random.randn(100,3), columns=['A','B', 'C'])
df['X'] = np.random.choice(['Alpha', 'Beta', 'Theta'], size=100)
pd.tools.plotting.andrews_curves(df, 'X')
```

![so](https://f.cloud.github.com/assets/199813/1434164/502f6770-410f-11e3-932b-c45889f5bddc.png)
 thanks for posting that - this looks good to me (going to try to pull down and see if I get the same thing) - anyone have any comments?
 Looks great now; thanks! 

Let me know if this needs to be a separate request, but consider: 

```
plt.ylabel(r'$f_x(t)$')
plt.xlabel('t')
ax.xaxis.set_ticks([-pi, -pi/2, 0, pi/2, pi])
ax.xaxis.set_ticklabels([r'$-\pi$', r'$-\frac{\pi}{2}$', '0', r'$\frac{\pi}{2}$', r'$\pi$', ])
```

Everyone loves properly labeled plots. 

![so_andrewscurves_03](https://f.cloud.github.com/assets/324454/1441531/b9485062-41ac-11e3-83a2-5efb37f456ce.png)
 Can you post another image? :)
 Let's keep this to the colors for now and can deal with labeling
separately.
 Just updated my last comment with an image. :)  I think having the grid lines in multiples of pi makes a world of a difference. 
 The colors change is minimal enough that I'm willing to merge it (and it seems clearly correct). We're in the middle of putting out a release so, in terms of changing labels or ticks, I'd prefer to hold off on that for a week or two. You can put up a separate PR that we can consider though. 
 It looks like you have Travis set up but that it didn't build this. If you think Travis is set up can you run `git commit --amend -C HEAD && git push --force` to get Travis to notice the build? 
 I hadn't enabled it to build.  Travis is running now.
 And it looks like it built my master branch (which is just master).   
 Need to check out the `fix_andrews...` branch
 Sorry, I am really confused by your travis set up (I am used to mpl where PRs are auto-magically built by the matplotlib travis account).
 you did it :)

If there's a way to do that in Travis, we probably should - that would be much easier for contributors.
 Can you add a quick note to release.rst? After that I'll merge.
 Done.

Squashed the commits.
 If you could, could you change the commit name to something like: "BUG: Fix color selection in andrews curve" and then you can put "fixed bug introduced in b5265a5
which change from picking a single random color for the class to
picking a random color for each line."

In the commit message?  Makes it easier to follow what's going on.
 like that?
 exactly. once it passes Travis I'll merge - thanks for the final cleanups!
 thanks!
