Graphcat

* Fix a bug marking failed tasks in static graphs.
* Suppress unnecessary updates using graphcat.passthrough(), graphcat.delay(), and graphcat.raise_exception().
* Improve consistency throughout the regression test suite.
* Make it easier to display customized graph diagrams.
* Static graphs emit the on_cycle signal when a cycle is detected.
* Expose standard task function arguments in expressions, but give domain developers the ability to override or remove them.
* Expression tasks sometimes create redundant implicit dependencies.
* Deprecate graphcat.execute() in favor of graphcat.evaluate().

* Expose the `rankdir` attribute when drawing graph diagrams.
* Alter graph diagram appearance based on graph type.
* Added a "User Guide" section to the documentation.
* Added graph.streaming.StreamingGraph.
* Greatly reduced code duplication among graph types.
* Calls to set_task() only mark the task unfinished if the new callable compares unequal to the old.
* Add support for visualizing performance data in graph diagrams.

* Cycles are detected during dynamic graph updates.
* A new signal notifies callers when cycles occur.
* Static and dynamic graphs behave consistently when tasks fail.
* Moved graph drawing into a separate module, so callers can customize graph diagrams.
* Added graphcat.common.consume() task function, for debugging dynamic graphs.
* Made pygraphviz an optional dependency, instead of required.
* Missing optional dependencies cause runtime failures, instead of failures at import.

* Introduced graphcat.DynamicGraph, which executes a computational graph with dynamic dependency checking.
* Introduced NamedInputs helpers to provide a cleaner / more consistent API for accessing task inputs.

* Deprecated graphcat.Graph, and added graphcat.StaticGraph instead.

* Added graphcat.PerformanceMonitor for evaluating task performance.
* graphcat.notebook.display() can optionally hide nodes that meet some criteria.
* Corrected typos in setup.py and release-notes.rst.