adds cursor()

Related to https://github.com/lmccart/p5.js/issues/34

`cursor(type)`
`type` can be a constant name defined in the original Processing reference ( http://processing.org/reference/cursor_.html ): either ARROW, CROSS, HAND, MOVE, TEXT, or WAIT
`type` can also be a String with a relative or absolute url or filename, or any valid value for the css cursor property.

`cursor(url, x, y)`
This version of the function only works with an image `url` (String), and unit-less x and y values. The current browser implementation is limited: only positive values under 32 are accepted. See: https://developer.mozilla.org/en-US/docs/Web/CSS/cursor

Notes:
This function does not support PImage to avoid a big overhead for very little extra-functionality. The user can easily emulate that cursor behavior by hiding the cursor with noCursor() and drawing a PImage at `mouseX` and `mouseY` coordinates.