arrow

ARROW-577: [C++] Use private implementation pattern in ipc::StreamWriter and ipc::FileWriter

This patch also includes some code reorganization -- I moved the reader and writer classes to their own headers/compilation units. I also moved the stream-to-file and file-to-stream executables to arrow/ipc

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

Closes #351 from wesm/ARROW-577 and squashes the following commits:

98c32d2 [Wes McKinney] Only build file/stream utils if ARROW_BUILD_UTILITIES is on
c5fa43f [Wes McKinney] Refactor to give make stream and file writer implementation details private in public ABI