Added convenience method to find out whether individual nodes contain failures
Slightly changed the output of the `experiment-context` plugin

The experiment start and end time tags are now called `<time-start>` and `<time-end>`, respectively. Also, the `experiment-shutdown` event will trigger saving the current time as `time-end`. If this was not called throughout the experiment before exporting, the export time will be used for this purpose instead.
Set success only if no failures are available in a node when ending its context

This fixes a bug in which an `end-context` event would overwrite any `success = false` states in any node that was set by `add-failure` before. Now, the nodes are properly marked as successful or unsuccessful (also in the .dot output, marking unsuccessful nodes with red lines).