Changed addNode methods to underscores, made names imply private and updated self._node references
Added test for invalid Node instance in add
Added test for invalid Node instance in add
Merge branch 'mohammed' of https://github.com/tmmurali/hypergraph into mohammed

Conflicts:
	tests/test_add_node.py
Fixed bug in get_node_names
Added test for get_node_names and get_nodes_by_name
Merge pull request #13 from tmmurali/mohammed

Mohammed