Allow for empty matches

[wergstatt]

The following example would fail:

val prefM = Map( 1 -> List( 2 ), 2 -> List( 1 ) )
val prefW = Map( 1 -> List( 2 ), 2 -> List( 1 ) )
val alloc: Alloc = Map( )
GaleShapleyState( prefM, prefW, alloc ).matches

The problem should be solvable by excluding unmatched boys that have an exhausted proposal list.

• [x] Add code example to the set of tests.