Provide new evaluation function leveraging latest evaluation api endpoint

[grabnerandi]

The current function sendStartEvaluationEvent sends a start.evaluation event to keptn usin the events api endpoint.
Since Keptn 0.7.3 we introduced a new endpoint /project/{projectName}/stage/{stageName}/service/{serviceName}/evaluation which provides better and easier handling of evaluation timeframe, start & end timestamps.

I suggest we introduce a new Jenkins Library function called "evaluation" that takes the same parameters as the new API endpoint and simply calls that endpoint to start the evaluation