Board now has relationship with blocks + Layer completion

There is now an array representation of the board spaces, which holds
the 'block' objects (instead of a list). This way you can quickly access
a block at a given position on the board. The blocks fall in a
contiguous manner to cover each space on the board. When a layer fills
up it is destroyed. (At the moment only the bottom layer of bricks can
be destroyed).
Layer complete bug fixes

now any layer of bricks can be destroyed (although bottom layer is only
the only layer that can be completed as the blocks are all the same
size).

TODO: make above layers fall down when a layer is destroyed
blocks only react to a collision with things below them
fragment cube (not important, experimental) + block (different box collider)
Merge pull request #1 from petrotiurin/Ricky---layer-complete

Ricky   layer complete