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
