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ARROW-1969: [C++] Don't build ORC extension by default

- Turns off building optional ORC extension by default
- Fixes travis builds to turn on ORC extension for a few branches
- Adds trivial import test to python build
- Adds documentation on how to build optional ORC extension

Author: Jim Crist <jiminy.crist@gmail.com>

Closes #1457 from jcrist/orc-off-by-default and squashes the following commits:

fc9898d8 [Jim Crist] Document how to build ORC integration
950ae38e [Jim Crist] ORC integration is off by default