v0.1.4: node_info.depth property

Add node_info.depth property

v0.1.3: graphviz similarity visualization & graph/node attributes

readme: update versioned figures commit hash

update to 13de9d63ac9d08ffc1e25ee80e912c611b990473

v0.1.2: support ontologies with multiple roots

Node_Info error if node does not exist

closes https://github.com/related-sciences/nxontology/issues/5

v0.1.1: add_node & add_edge support

safer add_node & add_edge methods

networkx add_node silently updates existing nodes.
networkx add_edge silently addes missing nodes and updates existing
edges.

Add NXOntology methods that raise errors for these dangerous
situations.

v0.1.0: initial release

update readme for PyPI