Add sort_columns parameter to plot to allow unsorted plots

[danbirken]

I was using pandas for the first time and when plot()ing I noticed it was sorting the columns alphanumerically.

I felt like the ordering I had defined in the DataFrame was a pretty decent proxy for how I wanted the subplots to be ordered, so I think this should be an option so I can over-ride the sorting and have the plot go in the direction specified.

[adamklein]

Looks good to me, but as it touches the API I'll let Wes have a look. Thanks for the pull req

[wesm]

Looks good to me

[danbirken]

Awesome!

Also I didn't want to push my luck too much, but is there a reason that the default is sorting columns in a plot? It certainly is a nice feature to have and I imagine for a lot of datasets it is a really helpful feature, but as a first time user it was strange to me that my plots were showing up in a different order than they were in my dataframe, especially given how wonderfully easy it is to re-order and take subsets of a dataframe.