ENH: axvlines - boolean option to parallel_coordinates plot
Determines whether vertical lines will be added to parallel_coordinates plot. I was using parallel coordinates to make a seasonality chart - plotting years on top of each other (days/months of year on x-axis). With that many xticks, the vertical lines were overwhelming. It's possible to remove the lines after the fact, but without specific line identifiers, it isn't a straight forward process. However, preventing them from being added is a one-liner...

@TomAugspurger ?
 @TomAugspurger ?
 Sorry I meant to reply earlier.  I'll take a look soonish. I'm a bit hesitant just because our parrallel coordinates habdling isn't the best. But I'll get back on this
 @bjacobowski ok, sorry it took me so long to get to this.

Could you post some code with small example of what you were doing? Also a before and after picture would be great.

We'll also need a test in `pandas/tests/test_graphics.py`. Just something simple like the length of the `.lines` attribute on the returned axes being equal to the number of lines (so no extra `vlines`). Also a test that the vlines are added (by default).

That said pandas is trying to move its focus away from plotting since there are so many great python vis libraries. This is so minor that I'm happy to accept it. But if you're interested you could see if seaborn or bokeh or ggplot would accept a PR for parallel coordinate support.
 @TomAugspurger Here's a basic example:
![parallel coordinates - axvlines example](https://cloud.githubusercontent.com/assets/7805755/4845197/4ca2e9c6-6042-11e4-92ed-a696d38aa952.png)
[seasonality.py](https://gist.github.com/bjacobowski/6c6d1ad0af8d50694765#file-seasonality-py)
 @TomAugspurger what are we doing with this?
 Let's get it merged with the understanding that any proper implementation of parallel coordinates should go into seaborn or ggplot or bokeh.

@bjacobowski would you mind adding a release note to doc/source/whatsnew/v0.15.2?
 @TomAugspurger done
 @bjacobowski can you try to rebase against master and squash this down to one commit. For some reason it's trying to commit the entire whatsnew/v0.15.2.txt. Checkout [the wiki](https://github.com/pydata/pandas/wiki/Contributing) or post if you have any trouble.
 @TomAugspurger that work?
 Perfect. Just going to wait until Travis is green.
 Thanks @bjacobowski 
