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ARROW-961: [Python] Rename InMemoryOutputStream to BufferOutputStream

Better API naming consistency with C++

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

Closes #690 from wesm/ARROW-961 and squashes the following commits:

85b352c [Wes McKinney] Add more graceful deprecation warnings for renamed classes, test suite
69a99cd [Wes McKinney] Fix Cython compilation
a15910a [Wes McKinney] Rename InMemoryOutputStream to BufferOutputStream