Backport of php5.5 fix for #11.

Conflicts:
	src/Gliph/Graph/AdjacencyList.php
	src/Gliph/Graph/DirectedAdjacencyList.php
	src/Gliph/Graph/UndirectedAdjacencyList.php
	tests/Gliph/Graph/AdjacencyListTest.php
Flesh out docs on AdjacencyList::walkSplos().