NF - Text numeric values for ha and va

Hi,

I added a quick patch to allow numerical values for text alignment.
It now allows a number that is fraction of text hight or with and is measured relative to bottom or left. So
ha = 0 == 'left'
ha = 1 == 'right'
va = 0 == 'bottom'
va = 1 == 'top'
va/ha = 0.5 == 'center'

the cool thing is that values outside [0,1] allow you to some defined distance from a line or symbol that remains well-measured even if you interactively scale the figure.

Can you please add this to MPL?

(also fixed some typos in typo doc)

OK, I try this pull request, I hope this is OK and I don't bother not too many people by maybe doing this somewhat out of line.  

-Alexander