implemented some responsiveness

[emmanuelle]

@ycaokris @sstripps this PR implements two things:

• if canvas width is not specified then it takes the full width of the parent element (closes partially change canvas width to allow for mobile responsiveness #36, the canvas does not resize when you change the size of the window but at least you can now adapt to different screen sizes)
• you can pass a height for the canvas (closes Unable to customize height for Empty canvas #37). This is overriden if a background image is passed, in order to keep the aspect ratio of the image.

Feel free to fetch this branch and give it a try!

[hobob0t]

any reason this isn't merged yet?