More illuminants

I added support for more illuminants. The color conversion functions such as 'xyz2lab' now can be used with two extra parameters: illuminant and observer. If no parameters are specified the functions behave as before my changes (they default to the d65 illuminant and 2 degree observer angle). This to ensure backwards compatibility.

In addition I created tests for the new conversion features.
Actually:

```
illuminant : {'A', 'D50', 'D55', 'D65', 'D75', 'E'}, optional
```