0.4.0 release

[asfimport]

We should target the next release around the week of May 29. We are accumulating some bug fixes and small improvements already, so we shouldn't delay in getting these out. We may want to push for some more integration tests and compatibility testing, so please attach JIRAs here to help with planning and priorities

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

Related issues:

• [Doc] File format documents incorrect schema location (is blocked by)
• ArrowInvalid: Invalid: Python object of type float is not None and is not a string, bool, or date object (is blocked by)
• Python: Table.to_pandas leaks memory (is blocked by)
• [Python] Reconstruct Pandas DataFrame indexes using custom_metadata (is blocked by)
• [Python] Rename InMemoryOutputStream to BufferOutputStream (is blocked by)
• [Python] Fix --with-parquet build on Windows, add unit tests to Appveyor (is blocked by)
• Python: Account for library versioning in parquet-cpp (is blocked by)
• [C++] Remove Array::Validate virtual function and make a separate method (is blocked by)

_{Note: This issue was originally created as ARROW-1014. Please see the migration documentation for further details.}