Add new API method "filter"

[artemdudkin]

I realized that current API cannot handle use case "get partial tree according to filter" to allow work not only with nodes somewhere in the meddle of the tree without parent nodes, but with all nodes from root with all parents, but excluding parents that does not fit given filter.

Method "get" should not manage banches at items of result set (as it does now), but only cut some items from result set array.