Real asynchronous file operations with asyncio support.

Features:
* Since version 2.0.0 using caio, which contains linux libaio and two
  thread-based implementations (c-based and pure-python).
* AIOFile has no internal pointer. You should pass offset and chunk_size for
  each operation or use helpers (Reader or Writer). The simplest way is to
  use async_open for creating object with file-like interface.
* For Linux using implementation based on libaio.
* For POSIX (MacOS X and optional Linux) using implementation based on
  threadpool.
* Otherwise using pure-python thread-based implementation.
* Implementation chooses automatically depending on system compatibility.