Initial reverb implementation

Respectfully, I disagree with this approach. Using a cmake file in interface to include a subset of the source files from an external source breaks encapsulation. It's also a surprising implementation that, as you note, will at a minimum require a new environmental variable for every builder of hifi. 

**I recommend we build an [lv2](http://lv2plug.in/wiki/ImplementingLV2) or [ladspa](http://www.ladspa.org) interface into hifi.** 

This would give us access to all audio plugins available for that interface, which could be awesome for developers who want deep control over the soundscape. It also preserves encapsulation and follows the standard conventions of including other external projects; Interface would only require plugins to build independently and follow the interface standard, as opposed to relying on their implementation details.

It's a significant project, but could be a very good worklist item.