DO NOT MERGE: When creating a stream, check for class before filename

Bug fix. When creating a `stream`, check for `class` before `filename` so classes can have the filename parameter. Unexpected behavior occurred in the following case:

	[watcher:aack]
	....
	stdout_stream.class = my.custom.LogHandler
	stdout_stream.filename = /my/log/path.txt

Despite specifying a custom class, **circusd** would always create a `FileStream`.