Add notes and links about legibility for text on maps.
Ensure that instances of L.Illustrate.Textbox are initialized with a minHeight and minWidth if not given in the options object (defaults to minHeight = 125, minWidth = 200).
Fix L.Illustrate.EditHandle#_latLngToOffset to return *half* of the minSize if the calculated offset is less than the minSize.  Eliminate L.Illustrate.EditHandle#_latLngToOffset and move code into L.Illustrate.ResizeHandle.
Require L.Illustrate.Textbox#setSize to reset _minSize if size argument is smaller than _minSize.
Add tests for L.Illustrate.Textbox#setSize.
Add tests for L.Illustrate.ResizeHandle#_onHandleDrag to make sure that the textbox resizes correctly in editing mode.