export the required canvas

I found it useful to export the Canvas object to the client so that they don't have to `require('canvas')` and load the whole module themselves. The reason I need that object is to use `require('canvas').Image` to draw something on top of the qr code ([like this](https://www.google.co.th/search?q=add+pull+request+to+issue+github&es_sm=91&source=lnms&tbm=isch&sa=X&ei=ZqVWU-i0KMmNrQen6YDAAg&ved=0CAgQ_AUoAQ&biw=1920&bih=968#q=line+qr+code&tbm=isch))