Add RetryingContext class (B)

[heri16]

Alternative implementation with support for AsyncRetrying.

See Issue #48

Example:

with RetryingContext(open) as open:
  with open('filename.txt', 'r') as file:
    ...

with RetryingContext(asyncio.coroutine(open)) as open:
  with (yield from open('filename.txt', 'r')) as file:
     ...

Flow: ContextManager(func) -> retriable_func(func_args) -> func_result -> ContextManager -> file
Async Flow: ContextManager(async_func) -> async_retriable_func(func_args) -> func_result -> ContextManager -> file

Outputs a context manager that returns retriable_func or async_retriable_func on __enter__.
Awaiting of func_result from the async_retriable_func is managed externally by the user.

[jd]

The with statement adds nothing again here. It's just equivalent to f = retry(someargs)(f).