Deprecating the trellis rplot module (GH3445)
Closes #3445 

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Start for #3445. Then the current examples should get a seaborn or ggplot alternative.

Questions: 
- explicitely refer to seaborn and ggplot as alternatives? (as there may be other packages that could feel neglected that way?)
- I now raise the warning on import of the module (`import pandas.tools.rplot as rplot`). The only thing is that the warning will be raised in the tests this way

> @TomAugspurger Did you already have some examples of how to do it in seaborn? I vaguely can remember you posted it once in an issue? 

@jorisvandenbossche I redid all the examples using seaborn. A quick search didn't turn it up, but I'll dig for it later.
 There are ways to both suppress and check for warnings in the tests, we have a couple of examples of it in mpl's test suite.
 I'm not sure ggplot is ready for prime time. I would probably only recommend Seaborn at this point.
 @shoyer yes, I could agree, as I noticed while trying to convert some of the examples to gpplot this was not easy (some of the functionality of rplot is not yet included in ggplot). I will update the PR
 I agree too: ggplot currently doesn't handle categorical data very well if faceting is used (actually that was my motivation for working on categorical data in pandas :-) ). 
 OK, I will remove ggplot for now from the explanation, and if @TomAugspurger finds his seaborn examples, we can add that. But @JanSchulz, if you want to add some ggplot examples when there is some progress, certainly do!
 @jorisvandenbossche @TomAugspurger let's do this for 0.16.0 (pls add a refernce in #6581) as well
 I made some examples of how to do it with seaborn, for each example in de rplot docs now: http://nbviewer.ipython.org/gist/jorisvandenbossche/103876bff4a5f42c9933#

@TomAugspurger Can you have a looks if this looks good? Or if there are better ways? (only for the last example (4 variables) I did not found a fully equivalent seaborn-way of doing it)

@jreback For the actual PR, I assume adding seaborn as a doc dependency is not that handy, so I better include all the figures as images?
 Joris, I remember talking to Michael about that last plot and it was agreed
that seaborn didn't have a way to do that plot directly. It was also agreed
that this is a good thing, since that chart is a mess :) Thanks for redoing
this
 I fully agree about that mess :-)

The rest is looking OK?
 This looks great to me!
 @jorisvandenbossche I think just need a release note in the deprecations sections
 @jreback ah yes!
