- Properly support 'after' alone
- Properly support both 'before' and 'after'
- Add optional 'reverse' parameter to sort messages oldest->newest to
1) provide a sorted result set for 'after'
2) give flexibility when using both 'before' and 'after'
This is a breaking change. Code will still work in Python 3.4 as-is but
if you use Python 3.5 you will have to change your code to the new
`async for` syntax as the older version is not supported in 3.5.
On the other hand, this comes with performance improvements if you use
Python 3.5 as it will lazily load 100 message chunks on an as needed
basis rather than loading all messages in one go.
