Support PEP 492 syntax

[gvanrossum]

In PEP 492 some new syntax is introduced: async def, await, async for, async with. These are live in Python 3.5.2. Once the fast parser lands (#1353) we should support these. See also python/typing#119; the tl;dr is that we should support

async def foo() -> str:
    return ''

[gvanrossum]

The first half of the implementation has been committed now: PR #1808 added support for async def, await, async for and async with. The second half of the implementation will hopefully follow soon: see PR #1946 .