Aiostream

- Issue 7, PR 8: add proper cleanup for `merge` operator
- Issue 3, PR 9: add `concat`, `flatten`, `switch` and their corresponding `*map` operators

- Add github context to the documentation
- Add the [chunks](http://pythonhosted.org/aiostream/operators.html#aiostream.stream.chunks) operator

Fix typo in docs, readme and docstrings.

First release