Update documentation of FixedCapacityQueue and LinkedFixedCapacityQueue to allow unmodifiable implementations
Add UnmodifiableFixedCapacityQueue to CollectionsIg
Add empty, singleton and unmodifiable FixedCapacityQueue to CollectionsIg
Add empty and singleton SortedCounter and split UnmodifiableSortedCounter in sorted and non-sorted
AbstractTree containsChild bugfix
Add in CollectionsIg synchronized versions of all interfaces
Add empty, singleton and unmodifiable BinaryTree in CollectionsIg
Merge branch 'wip/CollectionsIg' into dev