LAB support added

Resolves Issue #10 

`CIELAB` colorspace support added

``` python

img = Image("lenna")
labimg = img.toLAB()
labimg.show()

hsvimg = labimg.toHSV()

print labimg.isLAB()
```

`builtin` comparison added.

``` python

img = Image("lenna")
inv = img.invert()

(img > inv).show()

(img <= (150, 100, 200)).show()

(img == 150).show()
```

Added an example for fire detection based on LAB colorspace.