Refactored towers

Squashed commit of the following:

commit 23266c7e91304f27eb46f3d8072af71f5f4c171d
Author: Samuel Andersson <smulster@gmail.com>
Date:   Tue Feb 26 18:06:52 2013 +0100

    Added the last towers

commit 6b6b2d109d15c4f3e196ed0020b3542b9ab64c89
Author: Samuel Andersson <smulster@gmail.com>
Date:   Tue Feb 26 17:03:07 2013 +0100

    Make enemies go to center of tile.

    Apparently they were just visually moved 5 pixels into the tile, so towers
    ranging 1 square would lose target

commit 9726ea96eb45d689e740c2d7494e5c9011910020
Author: Samuel Andersson <smulster@gmail.com>
Date:   Tue Feb 26 01:02:24 2013 +0100

    WIP commit. Everything works.

    Just need to add the remaining towers

commit aa63d6b4a0f6698b205b5e0945f3a651268cb989
Author: Samuel Andersson <smulster@gmail.com>
Date:   Mon Feb 25 21:05:18 2013 +0100

    more wip. working compilation