Make parquet-cpp an optional dependency of pyarrow

[asfimport]

Currently pyarrow hard depends on the parquet support of arrow-cpp. This should be an optional dependency.

As there is no parquet-cpp release, this currently also blocks the Arrow 0.1 release.

Reporter: Uwe Korn / @xhochy
Assignee: Wes McKinney / @wesm

Related issues:

• Arrow release 0.1 (blocks)

_{Note: This issue was originally created as ARROW-282. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
With ARROW-296 completed, if the user does not have parquet-cpp installed, then pyarrow skips building the pyarrow.parquet extension