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ARROW-758: [C++] Build with /WX in Appveyor, fix MSVC compiler warnings

This will help keep the build clean.

cc @MaxRis

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

Closes #502 from wesm/ARROW-758 and squashes the following commits:

054c185 [Wes McKinney] Build with /WX in Appveyor, fix MSVC compiler warnings