Initial reverb implementation

This is not like the other external libraries. It does not build into a cpp library, and can't be linked in using cmake. It builds into an lv2 plugin (http://lv2plug.in), which requires an lv2 interface (http://lv2plug.in/wiki/ImplementingLV2). This is what I was trying to communicate in point (1).

As noted above, building an lv2 interface is a significant project that will add many new dependencies to Interface. While that has benefits, it may be overkill if all you want to do is add in a small, unmaintained library.