C++: Refactor given PARQUET-572

[asfimport]

I renamed the parquet-cpp C++ namespace from parquet_cpp to parquet

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-88. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 49
#49