arrow

ARROW-1194: [Python] Expose MockOutputStream in pyarrow.

This allows you to get the size of a record batch and schema through pyarrow by writing to a mock output stream. You can then use the resulting size to allocate an appropriately sized buffer to actually write to.

Example usage.

```python
import pyarrow as pa
import pandas as pd

val = pd.DataFrame({'a': [1, 2, 3]})
record_batch = pa.RecordBatch.from_pandas(val)

# Get the size of the record batch and schema
sink = pa.MockOutputStream()
stream_writer = pa.RecordBatchStreamWriter(sink, record_batch.schema)
stream_writer.write_batch(record_batch)
size = sink.size()
```

Author: Robert Nishihara <robertnishihara@gmail.com>

Closes #830 from robertnishihara/mockoutputstream and squashes the following commits:

4e15cd9 [Robert Nishihara] Expose MockOutputStream to Python.