Split library into two parts: DB-API 2.0 compatible and Asyncio style

まだ誰も着手していません。

I am very interested in using this library since type support in `psycopg2` is awful to work with. However, some projects still use Python 2.7, and will for another 4 years. Would it be possible to split `asyncpg` into two parts, one with an asynchronous API, usable with Python 3.5 and above, and one synchronous API that can be compatible across both 2.7 and 3.5 and above?

My proposal is a DB-API 2.0 ([PEP 249](https://www.python.org/dev/peps/pep-0249/)) compatible API that can be utilized by the `asyncio` part of the library. The benefits is that applications that do not need asynchronous calls can use the same underlying library as those who do. With this code that doesn't rely on co-routines can use the type support provided by `asyncpg`.

I haven't used this project yet, or dived into its internals, and don't know whether it is feasible at all to do this. I am just venting my ideas.

この issue では実装ファイルやテストが指定されていません。まず asyncpg の既存の asyncio API を確認し、提案されている同期 DB-API 2.0 インターフェースを PEP 249 および Python 2.7 との互換性と比較してください。これは探索的なアーキテクチャ提案であるため、maintainer はまず実現可能性、スコープ、測定可能な完了状態を定義する必要があります。

索引モデルが issue の本文から書いたものです。