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ARROW-348: [Python] Add build-type command line option to setup.py, build CMake extensions in a build type subdirectory

This also resolves ARROW-230.

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

Closes #187 from wesm/ARROW-348 and squashes the following commits:

3cdaeaf [Wes McKinney] Cast build_type to lowercase in case env variable is uppercase
74bfa71 [Wes McKinney] Pull default build type from environment variable
d0b3154 [Wes McKinney] Tweak readme
6017948 [Wes McKinney] Add built-type command line option to setup.py, build extensions in release type subdirectory to avoid conflicts with setuptools