Audio redesign

Audible components no longer track position and velocity; they are
dependent upon a Transform and Force for this data, thus allowing an
entity to act as an emitter.

Added rewind() method to Audible.

Submitting false or nil as the "al_buffer" argument to an Audible's
stream() method will advance the playhead but not load the buffered
data.

Mixer now updates the Listener upon invocation and with each iteration
of extant Audible components.

Mixer extracts spatial information from an entity's Transform and Force
components, if present, or substitutes an array of 0's of length 4.

Sound culling now updates the sound information and advances the
playhead, but re-enqueues empty buffers.  This should prevent playback
of culled (fully attenuated) sounds while still preserving their
playback should they enter audible range between loops.

Attributes component now exposes the methods of its underlying
properties cache with methods().