Execute should return an awaitable with AsyncioExecutor

The current graphql.execution.execute.execute implementation forces AsyncioExecutor to use loop.run_until_complete in wait_until_finished. This is a bit ugly.

It would be better to let the executor decide what is returned from the execute method. It can be given a function to call after the data is ready as an argument.

With this AsyncioExecutor could return an awaitable and execute would be used as one would expect

`result = await execute(schema, ast, executor=AsyncioExecutor())`

It could even have an argument to get the new way to keep compatibility with existing uses.

This would also make TwistedExecutor much easier to implement.