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.
Ok, i don't understand the behaviour of Python in this case. If I try `illuminants.get(illuminant).get(observer)` the code raises no exceptions. However if I use `illuminants[illuminant][onserver]` I get the expected results.

BTW, the tests run OK now, but I don't know if I should commit them again since some tests failed when using Python 3.