Really fix camera identification (fixes #21)

* Newer versions of ffmpeg write to stderr instead of stdout
* Added utility functions open_stdout_stderr, readall_stdout_stderr
* Using these, make sure we capture output of ffmpeg command used
  to identify the camera on OSX and (hopefully) windows

This works for me with the latest ffmpeg on OSX.  I don't have a Windows machine to test on.  @ihnorton, do you have time to test this?

Ideally, we'll get the camera names and parameters after #47 is merged, but hopefully this works as a stopgap.

Cc: @maxruby, @ihnorton