Scatterplot  Update for #3473
Rebased Code for Scatterplot targetting bug #3473

@cpcloud 

Thanks for your extensive comments. I tried to address all of them although the choice of some of the defualts around subplotting I simply copied from another Class (Line or Bar , I forget which) - so I didn't really think through the behavior there.

Can I bug you about git one more time? To make my changed I made a new branch, `scatterplot3`,  and manually re-added my few commits. I can issue anew pull request from this branch or I can try to rebase this branch, `scatterplot2`,  to master and add my `scatterplot3` commits. What do you reccommend?
 First things first:
1. [Hook up Travis-CI](https://github.com/pydata/pandas/blob/master/CONTRIBUTING.md#steps-to-enable-travis-ci)
2. Write a smoke test in `pandas/tests/test_graphics.py` as a sanity check (I didn't see one).
3. If you don't yet have `upstream` as a remote then add it via `git remote add upstream git://github.com/pydata/pandas.git`

About `git`: does `scatterplot2` have any changes that you need? Or are they all in `scatterplot3`?
 All the code is in `scatterplot3` except for the comments. I'll take a lookat some of the other graphics tests and make a new one for scatterplots. 
 If all of your code is in `scatterplot3` then do this:

``` sh
git fetch upstream
git checkout scatterplot2
git reset upstream/master..  # reset everything back to, but not including, your current upstream/master
git stash  # just in case you need something that you forgot :)
git rebase upstream/master
git merge scatterplot3

# if all goes well
git stash drop stash@{0}
git branch -D scatterplot3
git push --force
```
 Assuming you've set up your Travis-CI hook before doing all of that, it should subsequently run the test suite pretty soon after you run `git push --force`
 @jorisvandenbossche Thanks for the code review, however, this branch should be closed. The updated code is in #3473. This PR should be closed/deleted. 
 @zachcp OK!
