TST/CLN: legacy pickle testing

CLN:
- standardized testing data to live under tests/data (for that set of tests),
- added method ``util/testing.get_data_path`` to give you this path

TST:

- added ``io/tests/test_pickle.py`` and ``io/tests/generate_legacy_pickles.py``
to generate legacy pickle files and provide comparsions
just run ``python path_to_generate_legacy_pickles.py``

  it will create a version of pickle for that python version, arch, and system, and store in ``pandas/io/tests/data/legacy_pickle/VERSION``

- sample pickles for windows-64/2.7/0.10.1 and linux/2.7/0.10.1 and linux/2.7/0.11.0 added
- need to run this on various combinations to generate the reference library
- allows backwards compatibility testing for these pickles