Updated label.py to correct position of initial blank text

[kmatch98]

This is to resolve issue #80

Some background on label positioning:
There are two ways to specify a label's position, either by x,y or by anchor_point and anchored_position.
These two methods could be inconsistent, so I coded this so that setting anchored_position actually updates just the x,y locations. That way we don't have inconsistencies betwee the two. And the anchored_position getter just calculates the value based on the stored x,y value.
However, that means that the positioning of the label will depend upon the order of setting the position. For example, if you set x,y directly, then it will go to that position until you set anchored_position to another value.

In order to keep the anchored_position consistent, the text setter first gets the current anchored_position, then updates the text (in case the size changes) and then resets the anchored_position (which sets the appropriate x,y values).

To solve this issue #80, I updated anchor_point to allow a value of None. That way, if the anchor_point is None, then the anchored_position is also invalid. That will ensure that the x,y values are not changed when the anchor_point is not set and then the text is changed. I hope this is solution is generic enough to resolve most issues like this.

[kmatch98]

Sorry, it took me a couple of tries for your comment to sink in so I could fix this.
Deleted else, updated tabbing and recommited.

[tannewt]

Thanks!