add support for gauge panel type

[tonykoh]

Currently, if we try to create a dashboard using json model containing a gauge panel, it will be marshalled as a custom panel instead of a gauge panel. Due to json nesting of custom panel, targets.expr for the gauge panel will not be parse and no data is displayed on the gauge.

Proposal: Add GaugePanel struct in panel.go

[GiedriusS]

Sure! Feel free to add support for it. It would be nice to even have tests such as this for it: https://github.com/grafana-tools/sdk/blob/master/board_visual_test.go. Just a simple test to add a dashboard with that panel, upload it, and check if it appears at least.