Change desktop screen size to match mobile

Using iPhone 5 aspect ratio.
score and link length should be instance variables

I think it should be possible to have multiple “games” each with their
own gem grid. For example, you may have several games on the go at once.

Although this may not happen, this opens up the possibility to us.

Things which don’t change from game to game can still be accessed
statically.
Reposition various elements

* Ensure the GemGrid class actually draws itself with the origin at the
bottom left
* Put the button of the bottom of the screen
* Centre the gemGrid
Stop auto-replenish and detect level-end
Change background colour

To be revised later. At least now we can see the yellow shapes!

Only circle the last gem in the chain, and add a red line to show the
current path. Looks slightly less offensive than the weird orange
circle, but still to be improved.
Add in a predesigned level

Make it possible to specify a level when constructing a new GemGrid.
Also add a Gem constructor to choose shape and colour, otherwise the
previous is a little pointless.

We should consider the time investment into making a level editor. I
think they are pretty important for a puzzle game…