Always clear the image load timeout job.
Pass the correct object to image load callbacks.

Previously, the image object was passed on to onTileLoad regardless
if it loaded successfully or not. In case an image failed to load,
this would result in trying to draw a non-existing image onto the
canvas, which (at least in Firefox) throws a DOM exception and results
in the drawer being stuck mid-update.
Update documentation for Drawer#loadImage.

Additionally, improve name of the parameter passed to the complete
listener.
Merge pull request #15 from Ventero/imageload

Pass the correct object to image load callbacks.