FIX: check for cycles created by new connections

[klauer]

Closes #35

Checks cycles in scene.create_connection (i.e., for code mistakenly creating cycles) and also when the user completes a connection.

Adds exception type ConnectionCycleFailure

Adds some API niceties:

• Node is hashed by uid (now usable in sets/dicts/etc)
• Adds Node.has_any_connection, e.g., does node1 have node2 anywhere upstream or downstream of it in the pipeline?
• Adds Node.has_connection_by_port_type e.g., does node1 have node2 anywhere downstream of it in the pipeline?
• AddsNode.walk_paths_by_port_type to walk through upstream/downstream nodes. For example, if you have node1 -> node2 -> node3 and request to walk output paths from node1, it would yield (node1, node2) and (node1, node2, node3). This is used internally for the above two methods.
• Adds NodeState.output_connections, NodeState.input_connections to go along with .all_connections

[klauer]

Travis still not commenting somehow, but tests are passing: https://travis-ci.org/github/klauer/qtpynodeeditor/builds/672618906