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ARROW-442: [Python] Inspect Parquet file metadata from Python

I also made the Cython parquet extension "private" so that higher level logic (e.g. upcoming handling of multiple files) can be handled in pure Python (which doesn't need to be compiled)

Requires PARQUET-828 for the test suite to pass.

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #275 from wesm/ARROW-442 and squashes the following commits:

a4255a2 [Wes McKinney] Add row group metadata accessor, add smoke tests
75a11cf [Wes McKinney] Add more metadata accessor scaffolding, to be tested
e59ca40 [Wes McKinney] Move parquet Cython wrapper to a private import, add parquet.py for high level logic