add staticHeight option

[silverwind]

This is useful with the float option when one wants the grid to not automatically size down when moving items. Would have liked to have a similar option for width, but from the looks of it, width already never changes.

[coveralls]

Coverage decreased (-0.01%) to 70.514% when pulling 78f3cab on silverwind:fixed-size-grid into fa4f2d0 on troolee:develop.

[silverwind]

I fear I have to close it, it's not really complete. I have a branch that fixes some of the issues, but it's not really complete (one issue is that the collision check can place items outside the fixed height). Maybe I'll get this in a more complete state later.