Adds M_TAU constant for τ (2π)

This makes dealing with radians much easier:

```php
$r1  =     M_TAU;     # full turn      (360˚)
$r34 = 3 * M_TAU / 4; # three quarters (270˚)
$r2  =     M_TAU / 2; # half-circle    (180˚)
$r4  =     M_TAU / 4; # quarter-circle (90˚)
$r6  =     M_TAU / 6; # sixth-circle   (60˚)
$r8  =     M_TAU / 8; # eighth-circle  (45˚)

$radians  = $degrees / 360   * M_TAU;
$degrees  = $radians / M_TAU * 360;
$gradians = $radians / M_TAU * 400;
$percent  = $radians / M_TAU * 100;

$circumference = M_TAU * $radius;

$top    = [cos(            0), sin(            0)];
$right  = [cos(    M_TAU / 4), sin(    M_TAU / 4)];
$bottom = [cos(    M_TAU / 2), sin(    M_TAU / 2)];
$left   = [cos(3 * M_TAU / 4), sin(3 * M_TAU / 4)];
```

http://www.tauday.com/

We can't add `M_2_PI` because we already have that (2/π), nor can we add `M_PI_2` because we also already have that (π/2), and `M_2PI` would inevitably cause confusion. Plus, `(2 * M_PI)` and `M_2PI` both read equally badly - they don't really avoid the mental arithmetic.