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ARROW-459: [C++] Dictionary IPC support in file and stream formats

Also fixes ARROW-565

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

Closes #347 from wesm/ARROW-459 and squashes the following commits:

6a987b7 [Wes McKinney] Fix clang warning with forward declaration
8e0e6fb [Wes McKinney] Fix bug causing valgrind failure
dee044e [Wes McKinney] Review comments
7ac756e [Wes McKinney] Fix Python build
e5cec27 [Wes McKinney] Add some less trivial dictionary-encoded arrays to test case
acfa994 [Wes McKinney] cpplint
ef9dea8 [Wes McKinney] More dictionary support in FileReader. Simple test passes
cb04a41 [Wes McKinney] Refactoring. Remove FileFooter class in favor of private impl in FileReader
1cee0ff [Wes McKinney] More progress toward file/stream roundtrips with dictionaries
ae389fa [Wes McKinney] WIP progress toward stream/file dictionary roundtrip
6858e12 [Wes McKinney] Add union and dictionary to file/stream tests
d537004 [Wes McKinney] Add support for deconstructing and reconstructing DictionaryArray with known schema