Add Tree algorithms (skeleton)
Add unit tests for directed trees (100% coverage)
Add depth-first search for checking tree
Properly stop recursion and only check in isTree()
Throw Exceptions for parallel edges
Tests for parallel/loop/undirected edges (100% coverage)
Improve tree documentation
Remove Vertex::isLeaf() (see Algorithm\Tree\Undirected::isVertexLeaf())
Add Algorithm\Tree to CHANGELOG
Implement Algorithm\Tree\Undirected + 100% code coverage
Improve documentation
Merge remote-tracking branch 'origin/add-algo-tree'

Conflicts:
	CHANGELOG.md