Signalfx

Adds counters for errors during ingest and `reset_error_counters` to reset and
return those counters.

Added `get_dashboards`, `get_dashboard_group`, and `get_dashboard_groups`.

Fixed some bugs that tried to access a missing key

* Added `get_detector_events` method for getting events for a detector.
* Added new properties for computation populated by job info messages

Added methods for accessing the API functionality of retrieving incidents,
retrieving a detector by its ID, retrieving incidents for a detector by its ID,
and clearing an incident by its ID.

Also added preliminary support for `disable_all_metric_publishes` flag when
executing SignalFlow computations and removed an unsupported `Property`
datapoint attribute.

This release enhances the SignalFx pyformance package and extends the Pyformance
registry to support dimensional metadata. Please refer to the README and
examples for more information on changes to the pyformance package.