Prerender

this branch reduces time spent in tile.drawCanvas and tile.drawHtml by 50%

it also fixes some issues with navigator when the tile source tile size is greater that then the navigators smallest side.

it includes code to cache the tile prerendered on a canvas that optimizes copy time onto the larger canvas.

It also includes changes in openseadragons default settings.  the most important turns out to be the 

            minZoomImageRatio:      0.9,
            maxZoomPixelRatio:      1.1,

which reduced the point at which a tile transforms from one level to the next.  surprisingly by reducing this the max number of tiles drawn at any ui update is significantly reduced, ie, the number of tiles drawn at any given level has more negative impact than requesting new images for another level and drawing them.

I also added the recommended changes from dustmoo in issues #4, in whole or partially

finally the min level rendered was optimized which reduces ( for dzi at least)  the number of initial images from 33-ish to 9-ish per viewport.