Improvements for 0.0.3

[justinmanley]

TODO

• Allow users to specify in the options for L.Illustrate.Textbox custom classes to be placed on the <textarea>.
• Get weight and color for L.Rectangle created by L.Illustrate.Create.Textbox from the CSS styles on textarea elements. Will need to do this by creating an orphan <textarea> element with leaflet-illustrate AND user-provided custom styles, if any, and calling L.DomUtil.getStyle.
• Handle textbox zooming (link textboxes to the zoom level at which they are created and hide them when the user zooms out and they start to overlap with other textboxes).