Teoria version 0.2 - Chord voicings

Hey guys, here's something I've wanted to add to the library for a long time, but hadn't really thought out.

This PR makes `TeoriaChord` more note-agnostic (as the internal representation is changed from absolute notes, to intervals, relative to the root), and makes it possible to voice chords!

```javascript
var bbmaj = teoria.chord('Bbmaj7');
bbmaj.notes().toString(); // #-> 'bb4, d5, f5, a5'

// Change to voicing (to a delicious maj7 voicing):
bbmaj.voicing(['P1' ,'P5', 'M7', 'M10']);
bbmaj.notes().toString(); // 'bb4, f5, a5, d6'

// Reset the voicing to default (stacked in thirds):
bbmaj.resetVoicing();
```

This works, by having each `TeoriaChord` instance save two arrays: One for *all* the notes, which the chord includes (called `intervals`), and one which only saves the voicing, which can omit any note it wants (this is called pt. `_voicing`).

Only problem is: It brakes the API. Before `TeoriaChord.notes` was a property, now it's method. This is why I've bumped the version number to 0.2.0. Another thing is that it's possible to add intervals to the voicing which aren't part of the chord. Fx:

```javascript
var cm7 = teoria.chord('Cm7');
cm7.notes().toString();  // #-> 'c4, eb4, g4, bb4'

// This is a Cm9 with no root:
cm7.voicing(['M2', 'm3', 'P5', 'm7']);
cm7.notes.toString();  // #-> 'd4, eb4, g4, bb4'
```

So as you can see, it's possible to add a ninth in the voicing, although it isn't part of the original chord. I think this makes sense, as this is often seen in Jazz (putting 11th and 9th whereever you want), but I'm note sure that this is "technically" correct.

One other thing I'd like to discuss is how the `notes()` and `voicing()` method differ. Right now, calling `voicing()` with no arguments, just returns the current voicing. Maybe it should change so `notes()` always return *all* the notes of the chord. and `voicing()` instead returns the notes in the voicing?

I'm really excited about this change, and hope some of you guys will try it out, as I see a lot of potential in this feature (and especially, when using this along with something like #32 !).