Add tail_length exception, update example imports.

[kattni]

Added exception for tail_length on Comet animation - if it was too long before, it failed with a slicing error on the line containing animations.animate() which was incredibly cryptic.

No idea why I thought I needed to make the example imports and usage more verbose. Thanks @dhalbert for noticing this.

[kattni]

Looking into fixing the Pylint failure with now setting tail_length = None.

[dhalbert]

I forgot to note that you would revise the tail_length documentation, probably leaving out printing its value, etc.

[dhalbert]

pylint should notice that self._tail_length cannot be None. Maybe if we removed the intermediate variable (I have not tried this.

        if tail_length is None:
            self._tail_length = int(len(pixel_object) / 4)
        else:
            self._tail_length = max(2, min(tail_length, len(pixel_object)))
        self._color_step = 0.9 / self._tail_length	

[dhalbert]

Yay!