arrow

ARROW-618: [Python/C++] Support timestamp+timezone conversion to pandas

This was a massive pain. This patch brings us up to feature parity with the stuff that was in Feather. The diff is larger than I would like mostly from moving around code in `pyarrow/adapters/pandas.cc`. I suggest we split up that file at our earliest opportunity into the "reader" and "writer" portion at least.

The main work here was refactoring so that the data type for non-object arrays is computed up front (so it might be `timestamp('ns', tz='US/Eastern')`, then we use the visitor pattern to produce the right kind of array. This will also permit implicit type casts and conversions to integer from float because the type metadata is an input parameter.

Things are getting to be a bit of a mess here so we should do some refactoring eventually, and probably also add some microbenchmarks since this stuff is performance sensitive.

I also changed the C++ `pyarrow` namespace to `arrow::py` which will make it less painful to move that code tree to `cpp/src/arrow/python` at some point

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #375 from wesm/ARROW-618 and squashes the following commits:

4b18bfa [Wes McKinney] Fix rebase conflict
5bc3724 [Wes McKinney] Fix rebase issues
870986f [Wes McKinney] Refactor ArrowSerializer to not be a template and use visitor pattern using passed-in data type. Fix DatetimeTZDtype pandas logic. Arrow Change pyarrow namespace to arrow::py