Add option for home button to fill viewer (issue 127)

Adding functionality for https://github.com/openseadragon/openseadragon/issues/127 - option for home button to fill rather than fit.

- Added a option to pass to the OpenSeadragon constructor called homeFillsViewer, which defaults to false. If true, the home button will fill the viewer with the image, centered on the image's center, zoomed to fill the viewer at the image's smallest dimension, and clipped at the image's largest dimension. For example, a very tall, thin image in a 4:3 aspect ratio viewer will zoom so that the width of the image fills the viewer, and most of the height of the image is clipped.
Good catch!