Remove Graph::getVertexFirst()
Update references to Graph::getVertexFirst()
Remove Walk::getVertexSource() and Walk::getVertexTarget()
Fix invalid reference to Vertices::getVertexFirst()
Merge branch 'master' into remove-graph-getvertexfirst
Add removed `getVertexFirst()` to CHANGELOG
Merge branch 'master' into remove-graph-getvertexfirst

Conflicts:
	lib/Fhaculty/Graph/Walk.php
	tests/Fhaculty/Graph/WalkTest.php
Merge pull request #76 from clue/remove-graph-getvertexfirst

Remove Graph::getVertexFirst()