Better aac

This branch fixes a bug related to a creating a stream with a rewind buffer of 0, and adds an AAC stream for the standalone.config.

The last commit fixes a bug related to adding a preroll URL to a stream. When adding a preroll URL, StreamMachine attempts to get the stream key from the Stream's source by calling @stream.source.getStreamKey. However, when a Preroll object is created, it's source is actually another Stream object, and the source of *that* Stream object contains the stream key. Let me know if you have ideas as to why this problem exists only for a standalone.config and not a master-slave config.