PercussiveEnvelope = function(audiolet, gate, attack, release,
                                  onComplete) {
    var levels = [0, 1, 0];
    var times = [attack, release];
    Envelope.call(this, audiolet, gate, levels, times, null, onComplete);

    this.attack = this.times[0];
    this.release = this.times[1];
}
Simple attack-release envelope

**Inputs**

- Gate

**Outputs**

- Envelope

**Parameters**

- gate The gate controlling the envelope.  Value changes from 0 -> 1
trigger the envelope.  Linked to input 0.

@constructor
@extends Envelope
@param {Audiolet} audiolet The audiolet object.
@param {Number} gate The initial gate value.
@param {Number} attack The attack time in seconds.
@param {Number} release The release time in seconds.
@param {Function} [onComplete] A function called after the release stage.