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ARROW-243: [C++] Add option to switch between libhdfs and libhdfs3 when creating HdfsClient

Closes #108

Some users will not have a full Java Hadoop distribution and may wish to use the libhdfs3 package from Pivotal (https://github.com/Pivotal-Data-Attic/pivotalrd-libhdfs3), part of Apache HAWQ (incubating).

In C++, you can switch by setting:

```c++
HdfsConnectionConfig conf;
conf.driver = HdfsDriver::LIBHDFS3;
```

In Python, you can run:

```python
con = arrow.io.HdfsClient.connect(..., driver='libhdfs3')
```

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

Closes #244 from wesm/ARROW-243 and squashes the following commits:

7ae197a [Wes McKinney] Refactor HdfsClient code to support both libhdfs and libhdfs3 at runtime. Add driver option to Python interface