basic linear-gradient fill styles for the canvas

I can finaly include my simple changes.

I am not fully happy with the changes I had to made (the FillStyle is dependend of the starting-point of the gradient that must be the same as the region you want to fill) - but I had no idea how to do this in any other way. I wanted to keep the way canvas.hs was setup - that means setting the FillStyle as a Attribute (had to change this to write-only as the official canvas-api has no way to read from a FillStyle - only create and change it. It might be possible to recreate the backing haskell data structure but who is to say that this will not break in another canvas-implementation or browswer?)

I also added the beginnings of a very simple game (I want to implement a breakout-clone using this) - it's just enough to show a very simple linear-gradient (in the paddle) and let the paddle move with the mouse.

It is build just every other example.

Let me know if I should remove this or wait till it's really done.

Thanks for fixing the issues!