Started on excerpts and caching

Realized we should be proactive in solving the blocking problem: we do not want
play's limited thread pool to block on multiple concurrent ffmpeg extractions.
Unfortunately, I have not found a good way to solve this because java's
InputStream used by process in inherently blocking.  Furthermore, the same
problem already exists just reading object from the filesystem and accessing
the database because akka/scala/java are so far behind on asynchronous I/O.

On the other hand, we probably don't want very many concurrent ffmpegs running
on the system at all, so could just ignore this problem and bump up the thread
pool size a little and hope for the best.  I'm going to do some more research
asynchronous pipe/file I/O with actors first, though.  It should be a solvable
problem since there are all sorts of asynchronous network actors.