code style
swarm.announce() should always call callback

It’s important that calls to swarm.announce() always calls the callback
that it’s passed otherwise http announce/scrape requests
will hang (because res.end() is never called).

Also, since this swarm implementation is in-memory, we can get rid of
the callbacks to internal functions.

Lastly, fixed references to non-existent start() function.
Merge pull request #46 from feross/astro-fixes

swarm.announce() should always call callback