Tentative support for Pandas Dataframe with example using Titanic dataset

* Modified check_arrays function to leave Pandas data frames alone
* Modified is_multilabel function to support Pandas Series
* Added new DataFrameMapper class (inspired by @benhamner & mostly copied from https://github.com/paulgb/sklearn-pandas)
* Added new Titanic dataset to sklearn.datasets
* Added Titanic example to demonstrate DataFrameMapper use case

Looking for feedback since it is my first contribution to sklearn.

Thank you in advance!