Annotations

This pull request adds support for text arrows. Common code for the three drawing functions is now in one method.
Furthermore, a conversion for different positioning units is included in the merged text drawing. This allows to deal with second test figure from #394.

I think the conversion might cause some problems when the figure size in tikz differs from the figure size in Matlab. So in the future, a conversion to normalized or data coordinates might be better. 
This is just a tiny thing: but is there a reason why you use inches instead of SI units (e.g. centimeters)?