Improve API

[neocotic]

I've realised that my naming choices for the API probably wasn't the best it could have been as generateCanvas and generateImage are quite long and since these functions may be generating a QR Code on an existing <canvas> or <img> element it can be quite misleading.

For these reasons I'm considering renaming the functions to simply QRCode.canvas and QRCode.image. I'll only deprecate the existing functions for now as to provide backwards compatibility and I will possibly remove them in a future release.