ResultSaver should create intermediate directories

**Peter** ([Bug 55099](https://bz.apache.org/bugzilla//show_bug.cgi?id=55099&redirect=false)):
A FileNotFoundException is thrown if you use a relative path as 'Filename prefix' and a directory doesn't exist.

jmeter.reporters.ResultSaver: Error creating sample file for PicURL java.io.FileNotFoundException:
at java.io.FileOutputStream.open(Native Method)
at java.io.FileOutputStream.<init>(Unknown Source)
at java.io.FileOutputStream.<init>(Unknown Source)
at org.apache.jmeter.reporters.ResultSaver.saveSample(ResultSaver.java:188)
at org.apache.jmeter.reporters.ResultSaver.processSample(ResultSaver.java:150)
at org.apache.jmeter.reporters.ResultSaver.sampleOccurred(ResultSaver.java:140)
at org.apache.jmeter.threads.ListenerNotifier.notifyListeners(ListenerNotifier.java:84)
at org.apache.jmeter.threads.JMeterThread.notifyListeners(JMeterThread.java:783)
at org.apache.jmeter.threads.JMeterThread.process_sampler(JMeterThread.java:442)
at org.apache.jmeter.threads.JMeterThread.run(JMeterThread.java:256)

OS: All