Add ability to annotate graphs, save the annotation with the data

[discogestalt]

Initially would comprise the ability to manually annotate graphs and have the annotations saved so that when the metric is graphed again later the annotation will appear.

[discogestalt]

From adamsteffes: "It's helpful to know when code changed. We'd like to have our build system annotate graphs when deployments succeed." (Issue #36)

[adamsteffes]

Along similar lines, we would like to be able to annotate graphs with an alert threshold from our monitoring system. For example, if we set a 85% threshold on a CPU metric, draw a horizontal line at 85% in the graph.