Initial reverb implementation

I understand this comment to mean that we shouldn't include any of the gverb source in Interface, but rather require building and installing it's source project (https://github.com/swh/lv2) and then link against that. That's possible, though it adds a couple new complexities:

1.) Creating an lv2 or ladspa host in Interface, as the lv2 project builds into just the plugins. 
2.) Taking over support for the lv2 project, as it's not being actively maintained or developed.
3.) Several new dependencies in Interface to support #1 (at a minimum, fftw with 32 bit float support, pkg-config, and a lv2 host library such as http://drobilla.net/software/lilv/ which will have it's own dependency graph).

On the positive side, building an lv2 host into interface would let us use any lv2 plugin.